Don't listen to your friends blindly on this. It depends. If you accepted it, signed the paperwork and then 1 day before, you decided not to join, it could burn bridges with that particular group/manager. But turning down employment is totally acceptable in cases where you have multiple options and you decide what is best for you. It is totally ok to go for an interview and then decide if you want to join or not (if they do give you an offer).