The cadets I talked to at the Appointee day kind of shrugged and said most people don't really use them. I will probably get a cheap pair just to have in case, but if you want to save money, you're probably good without them.
On another note, I seem to get the feel that during BCT we will have about three and half thousand different worries, and money/finance will be right there toward the bottom. Considering that you are about to get a $400,000 education without any payment (with an additional salary!) there will probably be a few of us wishing we had spent the extra $20 for better undershirts or nicer boots or whatever it is. So, you might keep that in mind while preparing for BCT.