I don't try anything on when I'm at Sephora or any makeup counter. Mostly the things I have seen have been young girls who will walk right up and apply the tester (lipstick or gloss) straight to their lips. Ew! God knows who used that before you, and you have no idea what their hygiene habits are like or what germs they could have. I've watched girls do that at Sephora, and 2 SA's just stood by. They looked disgusted, but obviously not enough to do their job and say something to those girls.
I only swatch on my hand, and I always turn down the SA's when they ask me if I'd like to try something on. I have a pretty good idea of what works and what doesn't, and if it doesn't, I'll either return it, swap it, or sell it. Anything is better than using a tester, IMO.