Most on here will indicate anywhere from 78-82 is fine, though you will see ranges outside of this from some. I have limited saltwater experience but substantial biology experience and my own personal opinion is that while, yes most tanks can and do handle swings of temperature between 3-4 degrees, I prefer to keep things a bit more stable. My simple reason being that all things equal less instability to deal with leaves more energy for other tasks on a cellular level. I know that some may argue that in an actual reef the temps change wildly, but I would counter that my tank is not an actual reef and is missing many of the benefits, buffers and other unknown positives of the actual reef and any chance I can take to keep things optimal I will take. As far as actual numbers I believe that the higher end of the temp scale is preferable balanced against the dissolved oxygen content at higher temperatures. So for me I keep it between 81 and 82.
See, and you thought this was a simple question... LOL.
Hope this helps.