A general rule of thumb, if there is such a thing, is to normally not add reverb to the bass unless you're going after a special effect, i.e. a bass solo section. I've had some tracks where I've used a gate verb (think '80s Phil Collins) to the kick drum as a cool effect, but not the bass guitar. I've fiddled around with this in the past and all it really does is muddy the low end in a mix and hendering the clarity of the bass in the mix, though there are exceptions if that's the sound you're going for. As for adding reverb to an entire mix, I suggest you leave that up to a mastering engineer, unless you're doing "home mastering" for demos and such. Everything here is subjective and depends on what you're aiming for, but yes, ME's do utilize reverb and other effects as deemed appropriate (though I certainly am no ME by any stretch of the imagination).