If they're already on there, I'd leave 'em. No harm, no foul (the old "if it ain't broke, don't fix it" thing).
If one blows out in the future, then yank 'em and try 'em bare. Same work, just different timing; and may not have to do it.
I have run gaskets on manifolds (one had an ever-so-slight warp to it and I didn't have time to get it surfaced), but all I used was a cheapie Mr. Gasket that I had left over and hanging on the wall. Cut away the excess to the shape of the manifold (I DETEST the look of full-length gaskets spreading exposed across the head), used a dab of copper sealer and it's been fine for 3 years now... But yeah, I won't use gaskets whenever possible.