In the UKA and other places I've been it's more the ukes choosing the nage. I.e. Sensei says "5 ukes please", or whatever, and there is a mad rush to get to your chosen victim. Of course people often go for friends and this can be preranged to some extent, but it's alwasy a bit of a lottery, and after a few uke changes you've usaly run out of mates
One fun thing I saw one year at a BAF summer camp was the examiners rearange the ukes behind the backs of the testees. They all knew that one of them had a big, scary Russian 4th dan, but until they turned around they weren't sure who. The guy that did, did a great test though if I remember rightly, though the look of his face when he turned round to bow was priceless.
Personally I love to take ukemi for friends, I think its an honour and an intimate thing to be involved with. Like being best man, but the stag do is after