Most of us get to class early and do warmup stretching since the class is only an hour long. Depending on who's in class, sometimes we stretch for 3-5 minutes at the beginning of class but not always. We always do warmups/mechanics the length of the dojo: ukemi forwards, ukemi backwards, knee walking, "floor crawls" and maybe one or two others.
As for Janet's ukemi-as-diagnostic: I discovered that myself a few months ago, since if my ukemi feels "wrong" it means (on me) that something's misaligned along my spine. I'm working on strengthening my back muscles & intercostals and they're better at holding stuff where it belongs, but I've also gotten good at twisting and pushing in the right places til my ukemi feels right again. If my ukemi feels wrong but I don't do something to realign myself, I always wind up getting hurt.