"Yield to the lesser power"
At least that's what I've been taught. So yeah, it makes sense that downhill riders yield to uphill riders....except... my personal motto is "yield to the bigger dork". In other words, if the rider heading down is waaaaay outta control, skidding and grabbing handfulls of rear brake, I get the heck out of the way. I figure learning to stop and re-start on an uphill and/or practice trackstanding and/or taking the more difficult line only improves my riding.
Ditto for riding my road bike on bike paths. Most of the riders there are families (with kiddos) and newbies and my "job" is to give them plenty of room.
I need a jersey that says what I really think: "I'm not yielding because I'm a girl; I'm getting the heck out of the way because you suck." :p