I'm a ways out, but I intend to do quarterly withdraws. Whether I will withdraw money for Q2 on the last week/day of Q1 or if I will keep a small cushion (6 mo?) so that I can push my withdraw dates a bit and spend from the cushion remains to be seen. If I do the latter, if would be with specific parameters. (If market is down more than x% from last withdraw day, don't withdraw until either i need the cash or the market is back to within 2%, replenish cushion over the next 2 withdraws--for example. But this looks suspiciously like market timing, and we know that's evil, so...)
Also, this is assuming there is no fee for sales/withdraws.