The enemies scale so I don't think it really matters. Also, Pinnacle is a series of training simulations so you can leave and come back anytime while you are working on it. The goal is to just finish all the sims in 1st place and you can play them all as many times as you want. There is no real way to fail it.
Play it as early as possible. Provided you are playing on a difficulty lower than Insanity you should be able to clear it with levels in the 20-30 range.
Finishing first in all sims opens up a special mission developed by Ahern himself - upon clearing that mission you will have access to an apartment that allows Shep to purchase high-level weapons, armor, amps/omni-tools, and mods at a discount (although the purchasing is hierachially randomized).