Depends on if you're copying a specific helmet or not.
For instance, my sandtrooper has black on the inside because it's copied from the "move along" trooper... the only sandtrooper with all black.
There's one trooper in ANH who has one white and one black mic tip.
And for my ESB bucket I'm leaving the insides white.
So it's up to you... but if you're copying a movie or trooper so some research from the movies to see if they were painted or not!
Measure twice, cut once, and ask 1,000 times!