From past experience with a family member, we always requested wheel chair assistance at the airport (SFO, JFK, BOS). It helped a great deal esp with security. I accompanied the family member till the gate with a gate pass from United obtained when checking-in.
on board, they typically gave her a bulkhead seat. never an exit (obviously). she always boarded first (this was before sfo instituted the red carpet etc.). one fa usually accompanied her on board and helped stow the crutches in the FC cabin (usually the coat cabinet).
this was ua about 12-16 months ago. i dont' know how things have changed since then.