Deborah Neville offers excellent insight into the various roles of your support team, especially the distinction between your manager (who helps shape your overall career strategy) and your business manager (who tells you when you can and can't buy that pink tour bus). It's always a huge issue with my clients when I discover that they don't have a business manager (or, at the very least, an accountant).
While it may seem like overkill for your baby band to hire its own CPA, you need to designate some neutral third party to keep an eye on expenses and to sit on your nest egg. The coffee container marked "band fund" won't help you when your drummer wants to spend $100 on flyers while your bass player grabs $75 for stickers. A business manager keeps everybody honest and focused.
And, as Little Richard says, always sign your own checks. While you want a business manager to ask smart questions that may keep you from spending money, you don't need to actually give them access to the cookie jar.