I usually don't change the reverb level within a performance, except to turn it OFF for speaking, so there should be a nice common ground sound that will work for most songs.
If you don't use the L&R out's, naturally - you are still in mono..... how come?
Pick a Hall reverb to start with, and turn the mix knob fully LEFT. (off) then say the word "check" while you emphasise the "Ch" sound ..... gradually turn the mix knob to the RIGHT till you hear the CH trail off for about 2-3 seconds. You'll find that if the input level is hot enough - you won't ned to turn the mix up too far. Certainly not as far as the 12:00 position. Should be more in the 8-9 o'clock range... depending on mic signal of course.
Basically, you just want to fill up the holes (smooth it out) it will add alot if done with taste or make it sound like a canyon if you overdo it.
It will also make it easier to sing some songs because you'll have a natural "flow" happening that makes some dead rooms hard to sing in. A nice "live" room is such a treat to sing in .... just don't make it sound like a tiled bathroom - that's good for the ego, but makes you sloppy in the long run !
At Temple, we always ran scales in the halls because of the natural reverb. Made us sound like a million !