It is definitely preferable that Bettas are kept in tanks that are 75-82*... this is the temp of their natural environment, and the temp that they are spawned and reared in. I'm just like Ashley - my bigger tanks (3-10 gallons) are heated, and my 1 gallons are not, but they are always at 70-74*. They are definitely happy, but the Bettas that I'm conditioning and the males that I'm growing out as show Bettas are kept in heated tanks - because, to me, it's obvious that Bettas thrive on the optimum temperature of their natural environment. My boys/girls with heated tanks are much more active and colored up than those in unheated tanks... of course, my Bettas in unheated tanks are still very active, but I see a clear difference. Hopefully, I'll soon have all of them in heated tanks. During the summer, my house is 80* so all of the Bettas are very happy!

Compare tropical temps for Bettas to raw feeding for dogs - both are the most natural for these animals, and they will (much more often than not) thrive on a natural environment.