Generally agreed with the above. I would do my best to ensure that the tank isn't rising above 84 F, though pretty much everything we keep in captivity can tolerate temps a few degrees higher than that for short periods of time without problems (hours to days, depending).
Exactly what your temp targets are depend, I think, a little on how tightly you can regulate temp. If the tank naturally swings a few degrees daily (which is fine and potentially quite useful IMO), as long as it is maxing out at or below 84 F (e.g., 79-80 F in the morning, 83-84 F at lights out), you should be ok. However, there could also be the potential for temps rising higher than expected if you get a heat wave while you're away for the weekend, or something of that sort. Rock solid temps that vary only slightly on a daily basis (e.g., 79-80 F) can be great, but corals and other organisms are likely to be hypersensitive to elevated temps in the event of equipment failure, and a spike in temps. For instance, corals that see a temp that varies from 79-83 F daily are going to be fine at 84 F (or even a bit higher for a short period). Corals that are kept at a constant 79-80 F may be significantly stressed with a spike to 84 F, and may well freak out and bleach or die at higher temps.
So, I think a little of both worlds can be useful: some temperature variation when you are around helps to condition the corals and other critters to cope better with an unexpected temp spike in the event of equipment failure or a heat wave, but you definitely want to minimize the chances for problems when you are away, as bad things snowball fast.
cj