Spending the winter in a home with no attic insulation will be a little unpleasant if you've never tried it before. Insulating the attic will make your home much more comfortable this winter, and save you money on heating costs. With an open attic as you describe, current code requirements in Michigan call for an R value of 49 in the ceiling. So personally I would insulate the attic before winter with the highest R value that you can afford, and put the new roofing material on next spring. Unless the roof is leaking, in which case you'll want to take care of the roof as soon as possible.