The other thing you can do is go ahead and toss them in oil, presuming you were going to do that to bake them anyway. That coats the cut surfaces and keeps them from darkening. Room temp is fine for a couple of hours or even more.

I have had situations where cut sweet potatoes will start to develop black spots - I would put them in some water that has some lemon or vinegar in it (mild, maybe 10 parts water one part lemon juice). I think its OK to leave out unrefrigerated. Hope that helps and I defer to people with mre experience...