Do you consider trying on nail polish before buying rude or do you just consider it sampling?
I am talking about just a small swish on less than half a nail, not painting all 5 nails on hand...
Are we talking from a tester, or opening a bottle which you may or may not purchase?
Testers are there expressly for that purpose. Try away!
Opening a fresh bottle isn't really okay. One, if you (general) have any sort of nail infection or debris on your nail, you risk contaminating the bottle. Snooks, you'll note that if you go for a manicure, they wash your hands first. Infection is a long shot, but still! Two, once the bottle is opened, the clock starts ticking. Granted, opened nail polish has a pretty long shelf life (about 2 years), but it's still not fair to other customers.