If you're planning a pre-fab wall system, you really need to square up the walls before installing them, or you'll have a big problem keeping it waterproof. Shims or sistered studs would work, sisters may be easier.
It is MUCH easier to square the walls if you want to tile them, too. The cuts and alignment of the tile just come out nicer. Getting it nice and flat when setting the tile can be done, but takes a lot more time and skill.