Hey,

I can only share my view. The patibhagga nimitta (counter image sign) that Ajahn Brahm advocates is a sure sign of deep concentration but not necessary to attain the formless jhanas.

Although ​​​​​​it would seem having first cultivated the nimitta probably does lead to deeper formless jhanas. I have never done so, however.
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Hi Adam, this isn't answering the question about whether or not it's required but from my own experience I couldn't get into the formless realms until I was working with the nimita or "luminous jhanas" as they're called in The Mind Illuminated. These don't come into play until around stage 8 when your concentration is really well tuned. When I was working in earlier TMI stages and accessing jhana through piti/pleasure they were fairly weak and I never moved past the second jhana. That's not to say it isn't possible, but the depth of jhana when acccessed through the nimita was a lot more stable and workable for me.

It depends on what you mean by higher jhanas. For the formless realms (j5-8), no. For VM-style jhanas, it seems to be. Here is a discussion. 

http://rc.leighb.com/more/Nimitta.htm