There's lots of variables to balance here. Your stock should be the first thing. If you look at where they're from, and not just the geographic location, but also the depth, that should give you a temperature range per livestock. If you find a temperature that sits in each species range, then that's a good starting point.
Then you need to look at the pros/cons of higher or lower temperatures. For instance, if your species almost all would do good at 80, but one shouldn't be above 76, so you decide to drop down to 76, what's the trade off you're making?
By lowering temps you're decreasing your evap, slowing metabolism a touch which can decrease desired growth of corals but also decrease proliferation of any undesirable algae, diseases and parasites. It won't stop the growth of any of these things, it will only slow them down a touch. You're also saving money running cooler (if you're not using a chiller to do so).
If you run hot, you've got the opposite, you're increasing evaporation requiring more top offs, which is a minor issue for most. If you're using your heater to obtain the higher temp (rather than just latent heat from the lighting or surrounding air) then you're running up your electric bill, you're also increasing metabolism which increases growth of everything, good or bad.
So in the end, your goal should be to have your temperature set where the livestock requires it, any tweaking after that point is just personal preference.
Also, you'll see lots of books like clka mentioned that say things in the rough area of 78-86, but what I always fail to see mentioned is how deep they recorded the temperatures. Things like encrusting plate corals tend to come from deeper than branching corals, even if they're the same species, and this leads to two different temperature requirements as the temperature is different at 10' from what it is at 60-80' or further down.