lbb

That's good advice. Although, you might end up getting some misinformation, if these ukes' perception of what makes a good uke has nothing to do with why they were chosen.

Just within my dojo, and just with one sensei, I've seen people used for ukemi for all the following reasons (and note that this is my conjecture, but it's based on ten years of observation with this sensei):

• Most senior student present
• Most senior student present who's not recovering from an injury that makes this ukemi a bad idea
• Student with a particular body type that helps illustrate what Sensei is trying to teach
• Student who is likely to screw up in a particular way that lets Sensei illustrate a point
• Student who hasn't been to the dojo much lately
• Student who HAS been in the dojo a lot lately, and has been working hard on the techniques that are "theme of the week"
• Student who just came back from summer camp or another seminar where Sensei taught/attended, and who saw some different ways of doing things that Sensei wants to teach
• Relatively new student who is starting to "get it" and needs a new challenge
• Student with a birthday today (yes, really, "birthday ukemi" is a thing)

...and that's just off the top of my head.

If you want to take ukemi, make the most of your opportunities with senior students. My sensei tends to walk around class while we're working on a technique with our partners, step into each pair and have each student take ukemi (which often leads to a digression and a new direction in the class). If your sensei does this, that's your opportunity to take ukemi from him/her, so make the most of it. If your sensei doesn't, ask after class. But make the most of the opportunities you have rather than fretting about the ones you don't have.