I think it is sometimes wise to choose your partners. Of course (especially for the beginners) it's important to work with different folks. BUT I for example sometimes especially choose larger guys to work with if they want to - I can more easily see my mistakes on them. Then again when I sometimes feel really crappy, I say so and pick (for the whole lesson) let's say a nice girl who is much smaller than I am and then we do soft techniques. Also I think it's wise before seminars and exams (on the lower levels) "to get to know" someone. Because an uke you know can be so helpful. I mean as I understand it, in the exams BEAUTIFUL work is important - that's why you should do it with someone you know, so it really becomes a dancelike harmony. On the other hand in the seminars it's important to have an uke you know because it can make the understanding of techniques easier and you don't have to worry about that the uther guy could start "this is not the way that ukes fall in our dojo" or takes you in a headpin when finishing an iriminage while you are just used to the plain and simple throw.