Depends too on why you're learning them. Improv is definitely one reason, and really the end result there would be learn'em then forget about it, i.e. judge by your ear, not by some theoretical "this is correct, this isn't". The other thing though is by working through all the modes you really get the scale under your fingers. If you simply stick to say C major from C to C, you'd be surprised how your fingers will mess up going from say E to E. Muscle memory is a funky thing.