From the Dom's side of the coin.

Personally, i would rather a sub tell me when they are upset or not in any mood to be a sub because, to be honest,i want the sub at her finest. I would rather take a brake, or simply not do a task, until she is in the mood then to do it badly because she is pissed that she has already had a bad day and now i am making it worse.

Now this is just my side, other Dom's may feel differently.