Here's what we do, maybe this example will help. At the end of the fiscal year (for us, July 31), any money left in our checking account becomes the beginning balance for next year. Let's say it's $4,000. We then project how much money we'll make on our fundraising for the next year. Let's say that's $12,000. We add those two figures together to get all the money we have to allocate for the new budget, $16,000. We treat that total as one big pot of money from which we build our annual budget.
In our PTO, we create a balanced budget, so every dollar of the $16,000 is allocated to some expense line item (we have about 20 different categories). That's not to say that we'll actually spend it all...we have atleast 3 different miscellaneous categories that might not get tapped, and a category for our bylaw-mandated carryover ($2000) so we'll never run completely out of money. However, we do balance income to expenses for planning purposes.
If you make a plan to raise more money than you expect to spend, you'll build up a balance that no one will ever enjoy. On the other hand, if you plan to spend more than you expect to raise, you'll have to cancel programs because there won't be enough money to cover their costs.
The only time we might pre-allocate money from one year to the next is if we were saving over several years for a long-term project (ex: playground), but we would still show the balance on our monthly treasurer's report. Even though the money is "off limits", it's still one of the PTO's assets.
Audit trail as I define it (I'm an engineer, not an accountant) is a path of information that can take you from end back to source for any money or financial transaction. For example, if you start with a cancelled PTO check written to a vendor and work backwards, you should be able to match the actual check to the bank statemetn, also to the entry in the check register, match that to the invoice, identify appropriate PTO authorizations that allowed the invoice to be paid, match that to the originating event such as a scheduled assembly or vote in the minutes of a PTO meeting. Conversely, if you have a cancelled PTO check and no substantiating paperwork or information to explain the check, you have no audit trail. Accountants (and volunteer auditors) love audit trails. So should PTO Treasurers.
Hope this helps, engineer to scientist.