Yes, the time to do Roth conversions to reduce your balance in traditional IRAs so as to reduce future RMDs its generally prior to age 70½. It's generally recommended that you convert up enough in one year to fill up your current tax bracket. The earlier you convert, the longer the investments have to grow tax free in the Roth IRA rather than tax deferred in the traditional IRA.
Regarding withholding, if you have state taxes withheld, deduct the state withholding as an itemized deduction, then receive a refund of some portion of the state withholding, the refund of the state withholding will be taxable.
