I too was taught only to fall if the nage makes me need to, don't be stiff, don't force resistance (with an unwritten "unless you're a pair of yudansha testing each other's response"), but there's one exception I'm curious about that I haven't gotten around to asking sensei about yet. Anything that involves a throw into rolling ukemi, you roll no mater what, even if the person didn't successfully throw you. I'm guessing this is partly for safety, partly for practice doing rolling ukemi well? It usually manages to still be obvious to the nage and the sensei if the throw wasn't good.
Ever seen someone pull out of a roll at the last minute and land on their shoulder? It is not a pretty site. A dislocated collarbone can take months to heal, if it ever fully heals.