I have the Viper and I have the 2nd, 3rd and if I did it right, 4th axis set (I DID NOT DO IT RIGHT...LOL) and yes, I use my level every shot.

In my back yard, it's a side hill, if I don't use my level, I drift shots right. (downhill) It's more noticable at farther distances and the steeper the hill. As I've said before, archers have a tendancy to cant the top limb down hill on side hill shots and will have a tendancy to shoot right.