Mine pretty much just drop eggs on the gravel, driftwood and each other, with one olive who seems to prefer crypt stems. Noticeable? Only if you're looking close. They don't bother me, but I can see where someone trying to create an impeccably aquascaped tank might go a little bonkers seeing them.
For their cleaning ability--I only scrape my walls for green dot algae once a month!--it's well worth the minor annoyance. If you're worried about eggs and don't particularly care to see much of your nerites--go for the olives. They stay smaller and spend most of their day tucked away. My zebras lay larger, brighter eggs and spend most of their day in plain sight-. I swear whenever I get out the camera to do tank shots there's guaranteed to be a zebra snail butt on the front glass.