Some of the "rules" I've found, at least around here:

1. Take the stall by the door, and if that's occupied, take the stall nearest the door. This means that the last stall (the handicapped stall) is usually unoccupied, and the stalls nearest it are cleaner, better supplied, etc. than all the others. As for the "advice" given above, I've rarely encountered wheelchair-using women using a public toilet (no kidding, please, I haven't!), and hence have found the extra room affords an ideal place to deal with extra-excretory activity (looking through my handbag, sneaking a quick toke, major costume changes, etc.) that might need more time and space than the usual. The stalls near it are my first choice, of course, and naturally, my spidey-sense is quick to detect anyone moving near that part of the room.

2. Never acknowlege smells, sounds, or anything else in a stall, but be quick to at least smile and give a word while near the sinks. This is a) to minimize embarrassment, since it's easier to pretend that you're here for something else that way, and b) to exchange intel, small favors, etc. out of range of menfolks and other prying eyes. The resulting low-level noise keeps the attention away from stall-dwellers and onto more intriguing topics such as "Is there lipstick on my teeth?" Middle-aged and above women are more likely to do this than younger, since they're more likely to have other activities to attend to (cosmetology, hair fussing, etc.)