First off, I wanna make sure that krypticrhacs understands there is a difference between isopods (aka rolly poly, wood louse) and collembolans (springtails). The size difference is pretty big. Springtails are roughly the size of a pinhead cricket and isopods are usually about 1/8-1/4 inch.
I've heard from a curator I know that isopods don't really do much for the diet. He swears he's seen them pooped out by geckos, still alive.
I doubt springtails would take over a terrarium. They do become numerous, and I'll admit that I've questioned whether a few eggs I found too late went bad because of them. I would be just as cautious with isopods. But if you think of it, you don't really want your geckos eating the same things that might be eating their feces.