First of all, sitting in full lotus position is a pretty tough thing if you're even remotely tight in the hips. It can also be murderous on the knees.
On the floor, I usually sit in 'burmese' style with one leg in front of the other. Sometimes I'll do a quarter lotus. If I sit on a floor, I use a zafu (cushion) which keeps the hips higher than the knees. If I sit in a chair, I'll use a cushion if the knees need to be higher. To combat the slouch, I think it's useful to pull the head up lightly and tuck the chin every so slightly. In martial arts training, while standing, there is often the image used of a string pulling the crown of the head up gently. If you pull your head up in this manner and also allow the feeling of the spine stretching, it can be helpful.
No easy answers, except go in with good posture, keep bringing the posture back when you notice any slouching, and be gentle on yourself. Time should take care of the rest.
As far as whether slouching is meditatively good or bad... I think it's only the latter insofar as it really can screw up your back given enough time. Apart from that, you could just pay mindful attention to your body/posture. It's all grist for the mill.
