I agree, 84, up to 86 is really fine for most betta fish. I have kept a few betta fish at this temp. Keep in mind a few factors though: the heat will cause your fish to have a faster/higher metabolism, and as such should be fed just a little more (don't overdo it!), and as such will pollute the water more, and as such should have one more water change per week (50%).
Also, long term at this temp will shorten the lifespan for the above mentioned increased metabolism. By how much? Hard to say. If your betta was going to live 7 years, maybe now it will only live 6 years. Maybe the higher temp will ward off an ich infection because you let the water go to long and he will then be able to live longer because of it! Plus you figure some flux in temp is normal was well, just make sure they are not fast/dramatic changes. That is when the fish have problems!