You may remember that you can make an RRSP contribution in a given year but carry forward the resultant RRSP deduction until a future year. This rule still applies even after you have matured your RRSP into a RRIF (or any of the other conversion options).
The RRSP deduction will reduce your taxable income in the year that you use it. So if you think that there will be a significant tax liability a few years into retirement, such as the sale of capital property that would result in a large tax bill, you could make your RRSP contributions but delay claiming the RRSP deductions until that year. In other words, you do not lose the ability claim the RRSP deductions once you mature your RRSP.