It fluctuates between 70 and 80, usually. I keep the humidity between 40 and 55% constant. I've read different accounts of the effects of temperature shifts of 10 degrees or more, but it seems to me that it better mimics the change in temperature between night and day. Some say its either beneficial or has no effect, others claim it can stunt the plant or cause other issues. As this is only my second grow, I really don't have a educated opinion either way. The first one turned out exceedingly well, so there's my only anecdotal account.
For me, I was thinking of using an IR thermometer to measure temperature delta between air and the top of the plant.. see if I might want to move the light one way or the other.
I just did exactly that yesterday, shifted the l8ght above my early 1-2 week old plants up 4 inches, leaf and soil temps are way more stable and manageble now.