I'm not aware of any good reasons, but I think GAAP standards say you can't recognize the revenue until the returns period ends? I think standard wisdom is to not have an official policy so as to keep the books simple (and maybe to avoid the downside I can't think of for returns periods) but use refunds pretty aggressively in support?
Your company should be on cash-basis accounting. Unless you have a need for accrual based. When you file corporate taxes, you list the method of accounting you are using.