When it comes to the jumpseat, the Captain owns the aircraft, but the ops agent owns the jetway, and there are courtesies for both.
As far as preboarding goes, I would never assume either way. I would ask the ops agent what they would prefer you to do. An offer to take a pre-board wheel chair down is a great way to score points with the ops agent and pre-board as well.
One issue with preboarding is that often times the Captain will be off on a food run. Most Captains will let the FO's meet and greet jumpseaters, but there are a couple who won't, ususally those who like to check the scab list first. Overall, SWA pilots pride themselves on the easiest Jumpseat in the industry, but you can't have almost 5 thousand guys without a handful of odd ducks. Also, because we are easy going on the jumpseat, we occasionally have jumpseaters who take that for granted and treat the privilege as a birthrite and act like they own the place.