in our dojo, we're taught to always take the fall, no matter what kyu or dan you are. you get practise with your ukemi, and nage gets practise with their technique.
we're also taught, as uke, to always give energy. though, we do exert control so as to not catch nage off guard or pummel through 'em with a tsuki of some sort.
i'll take the fall 90% of the time. the rest of the time i don't is because nage isn't correctly executing the technique. also, i appreciate a good "throw" to the ground, especially by someone who knows what they're doing.
also, in order to be a good nage, you should also know uke's role. especially if you are working with someone who's just beginning. ya won't be able to show/teach the technique unless you know both sides too.