I mean “quality” is subjective, of course. If you have a very specific way it needs to look then that’s harder to match in a final output. The general thing I do when final appearance must be a certain way is to crop a square inch of my image and run tests using that much smaller image before doing the full engrave. Saves materials and most importantly time.
As for materials, each material has its own quirks as to how it’ll interact with laser. That’s not a Glowforge thing, that’s a physics thing.
As for image prep check out #9: