Deanne,
Each customer I have worked with has different requirements for this. For example, I have one customer that constantly needs to go back and unapply/reapply pretty old payments. They use a cut off date that is 6 months prior, so they can easily do this. Other customers move everything to history right away, but then do use the unapply tool once in a while as needed. Yet others issue financials to the board and sometimes the bank that include details of their AR aging. In that case, they really don't want those things to change after the fact and thus don't want to unapply anything for a closed month.
I don't think there is a wrong or right, but I think the important thing is to be consistent and to keep running the Paid Transaction Removal so your open tables are not bogged down with unnecessary data.