I think as a matter of course and principle you'll want it to be as quiet as possible.
Think of it this way, the higher your noise floor the louder your sources have to be, or the closer you have to be to your object. You want the sound you're aiming for to be loud enough that its spectral (frequency) content is not being affected by the sound of the noise itself. This means that you could record something like a gunshot in a much louder environment than you could record the sound of turning a page in a book.
Gaining up will definitely bring up the source sound, but the noise is going to come up with it, so you want as much distance as possible (in terms of amplidtude) between the noise and the source. -50dB probably isn't too bad, but you're going to run into problems when you try to do delicate stuff like textures, closthes rustles, and things like that.