Do some searches on "heater", this debate has come up many a times.
I am now in the "no heater" camp. I had one go haywire on me, so I took it out and never put it back in my 75g. And I have not put one in since (I would have to look when I actually took it out, but feels like about 1.5 years ago). I haven't checked my temp in quite a while, but everything is thriving very well.
I think you are on the right track by taking queues from how your tank is actually behaving.