We always get to pick ours. Kyu tests are posted in advance and most dan tests are known about ahead of time so you pick your uke well ahead of the test. Even on the surprise dan tests sensei will tell the nage to pick an uke because they are testing in 5 minutes.
Michael Fooks wrote:
Murphy's Laws of Combat
if it's someone you're used to you're going to be fully prepared for exactly how they move.
#25: Tracers work both ways.
The uke for my last test was somebody I work with on a very regular basis. I know how he moves...and he knows how I move and what my weak points are. The bastard did his best to expose and exploit every single one...and I greatly appreciate it