When ever I do either  samatha or vipasanna. I notice so.etimes I feel good very  relaxed and 1 pointed. This always seems to happen when my breathing is shallow and  slow ,particularly  the out breath. Also there seems to  e quite a long pause between the out breath and In reach.
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When your exhalations are longer than inhalations, the concentration of CO2 in your blood rises. This is relaxing. One of the biggest obstacles to concentration is stress, so relaxing often improves concentration. It's why I always start with relaxation. I find a brief period of relaxing meditation can produce stronger concentration than a much longer period of other types of meditation.

Another advantage is that when you are relaxed you can notice dukkha arising more easily because it disturbs the relaxed state. An you can tell when you have let go of the attachment or aversion because you are able to return to the relaxed state.

Reason why I am saying this . I usually  just observe  breath. I am wondering if I should start guiding  my breath in that direction.  Generally buddhism isn't about breath exercises but observation. 

It is your playground, do whatever you want. "A Bikkhu breathing long knows that he is breathing long, breathing short he knows that he is breathing short"
Or something like that quote. Noticing all qualities of the breath is vipasanna and can be blending concentration with insight.....It can be a shortcut when you master the variations of manipulating breath to get to a good spot, then work from that point to the unknown cutting edge of your practice.
Good luck,
~D

If you look at the anapanasati sutta it really consists of several different types of meditations intended to be used during a meditation session. The initial ones are samatha type practices for calming the body, feelings, and mind, in effect preparing the mind for the later practices which are more vipassana type. So my advice would be to use the breath to develop tranquility and then when the mind is tranquil to do a more observational type of practice.

When you discover a practice that works for you I think that can be very meaningful, it personalizes your practice, it can lead you to insights in ways that shrink wrapped practices don't get close to, so I think you should use it, but it doesn't have to be your only practice.

I would agree that, generally Buddhism isn't about breath exercises, if we are contrasting it with things like yoga but there are certainly teachers who emphasize modifying the breath, and whole schools that focus on the specific mechanics of sitting and walking, for example. I also agree that Buddhism is about observation but I would add that it's observation in the context of learning about cause and effect. So if you change the way you are breathing and observe the kind of effect that has, that would count as Buddhism for me. Personally, I don't manipulate my breath because it doesn't seem helpful to me personally, but I manipulate lots of stuff when I sit, including posture, attention, attitude, digestion, time of day, etc., so I would not hesitate to at least experiment with manipulating the breath, if it was helpful, and use that as fodder for better understanding of how mental phenomena and mental states arise and pass away depending only on conditions.

Oh yes anapasati can take someone to enlightenment (so I'm told). Few buddhist teachers train in breath patterns  and such

Rinzai Zen is right into breath training, although they want it deep, not shallow.

Here is the beginning of a whole chapter of the breath in The Rinzai Zen Way by Meido Roshi.

MEDITATION—THE BREATH

Even though one may be hemmed in by worldly cares or tied down by guests who require elaborate attention, the source of strength two inches below the navel must naturally be filled with the vital breath, and at no time may it be allowed to disperse. This area should be pendulous and well rounded, somewhat like a new ball that has yet to be used. —HAKUIN1

ONE REASON FOR our detailed emphasis on zazen posture is that it allows deep breathing of great power to manifest. Practitioners have long observed that the quality of one’s breathing has a dramatic effect on both the energetic system and mind. Cultivation of the breath is therefore crucial, and its mastery is among the factors determining the depth and clarity of our meditation. To be more exact: it is by using breath and posture to unify the mind and energetic currents at the navel energy center (tanden) that a samadhi of the greatest depth and subtlety may be cultivated. This also reveals the means by which insight will ultimately be made to permeate the body. Luckily, breathing is a process that is partly under our conscious control. Thus we are able to train it, and there are many ways to do so. In general, it may be said that breath practices in Rinzai Zen are grouped into four categories revealing a progressive training:
1. Methods of establishing (or reestablishing) natural abdominal breathing 
2. Taking the latter as a foundation, methods of cultivating a trained breathing focused on the tanden
3. Methods of increasing the depth and energetic vitality of the tanden-focused breath
4. Finally, methods of lengthening, refining, and integrating the tanden-focused breath so that it is subtle and constant, in both meditation and daily activity 

It's a thing. It's not my thing, but it is a thing. 

The breath is a subtle and fascinating meditation object. It's automatically happening all the time despite being ignored by the conscious mind, and controllable in some ways, seemingly at times subject to the will and intent of the conscious mind. This is a window into how the mind operates in general - how experience is this weird combination of attention, awareness, will, intent, conscious, and unconscious. LIke meditating on any object, the nature of our experience can be slowly uncovered by a focus on these mysterious interactions. Therein lies insight.