It really is a matter of personal preference. None of the original props used mesh in the "teeth" holes. The highest-res pictures I have it's not clear if it's window tinting, a strip of black vinyl, some sort of smooth fabric (for some breathability), or what. The only clear pics of the interior of an original helmet that I've seen have whatever was backing the teeth missing.
I have some black panty hose that I cut up as needed for vents like this. It does a pretty good job of approximating the protective foam or fabric filtration layer in similar real-world applications. If you want something sturdier, you can always back that off with some window screen. But if something resembling on-screen appearance is important to you at all, I'd avoid visible mesh.
--Jonah