Depends on the quiver. I shoot a ligh Hill style longbow, so a heavy bow quiver like a selway really throws off the balance of the bow for me and I shoot like crap. But, I recently got a light weight Eagles Flight Archery quiver and although I still notice a bit of added weight, it doesn't affect my shooting like the Selway did. Yes, you should practive with the quiver on the bow to make sure it doesn't affect your shot, or so you get use to it while shooting.
I'm a firm believer that all quivers suck. They all have their disadvantages and I'm continually all the quest for the perfect quiver.