I will usually stay unassigned. My thinking is, the seats that come open at the airort may be better (bulkhead, aisles cleared by upgrades, etc). If you are unassigned, then they will have to give you something, and it may be one of these. Once you have a seat and want to change it, you become a one of those pesky annoyances that busy GA's don't like to deal with.

The exception is flights when I expect to arrive at the airport close to the check-in cutoff, so using OLCI is critical.

All that said, if I have a lousy seat, I'm pretty anal about checking frequently, and can almost always find something better than 31E in the day or three before the flight.