Im about to enlist, and Im gonna be getting e3 before I go based on college credits, he said if I complete my pre-BCT checklist I might be able to bump up to E4 but he will have to check regs on that. It might be worth waiting the year and a half and attending a junior college and amassing as many creds as possible (easy fun classes, its just the credit count that matters) for a year and a half, and being able to enlist at a higher pay grade.
John
EDIT: As far as income tax, uncle sam wants his cut unless he is actively serving in a combat zone is my understanding. I do know that you can finish your degree once you are in and get a diploma from the Army. Army provides 100% tuition assitence for schooling taken during your enlistment, and credits earned go towards promotion points for your next rank.