That's up to the rules of the coach line. However, I want to caution you- while there's little physical violence in Europe, there is a lot of pickpocketing, and back packs are an easy target. Do NOT put your money, wallet, passport, or credit cards in a pocket of the backpack. Theives slash them with razors and take the stuff without you even knowing. Also, be careful with the backpack in crowds. People turn suddenly and forget they have this big pack on their backs, and it whacks other people- sometimes in the face.
Nothing screams "tourist" quite like a backpack.