I just wanted to throw this in here, too...

We are assuming (and likely correctly, but still assuming) that the OP is talking about a cyclist on a road bike. I've been known to do road hill repeats on my mtb, simply b/c that's what I ride on a regular basis. My mountain bike (and please don't judge me for the quality of my bike) would only go 25 mph on a descent if I threw it off a cliff. In that case, I would NOT be "sharing the road" by taking the lane and riding my brakes down in front of a car. Which is why, if it was me on my bike, it would be a better idea to pull off into a turnout. Besides which, I've already demonstrated my aptitude in flying over the bars and I'd rather do that behind a vehicle.

Not to argue with any of the valid points that have been made here. Simply to illustrate that situations may vary. I think the point everyone has made is that we all have to judge our situation and do whatever we feel is safest.