it depends on your preference
I prefer back riveting, especially if your helper is inexperienced. All they have to do is put in a rivet and place the bucking bar over it while you back rivet. I think it also leaves a better appearance on the outside with less tendency to cause a depression from riveting from the outside. (If you do get a depression from riveting you can tap up on the shop head to reduce the appearance from the outside.)
The front skin is a different animal altogether. You can not back rivet a bunch of rivets, inside the gear towers and other areas if you already have the radios, etc installed. It is possible to drive all of the rivets on the front skin, but some find it easier to use blind rivets in hard to get to areas.
Shimming, as already stated, makes a much nicer final product if you have any low spots. I use aluminum flashing material for shims quite a bit, since it is cheap, very thin and virtually eliminates visible steps in the skin after riveting. Get the skin on as tightly as you can with the clecos for the best results.
A&P, Tech Counselor & Flight Advisor