As far as mileage and general wear goes, between 195-205* is where you wanna be. Going 215* or higher can give you some performance issues, but so can going below 185*.
I have to switch back and forth on mine and run a 195* T-stat in the cooler months and a 180* unit in the hotter months; when I run too hot, I get pinging problems, and if I leave the 180* in when ambient temps cool down, my gas mileage takes a dump. Of course, this is on an EFI setup, but on a carb'ed setup, it's going to be relatively the same. A cold motor is not going to run as efficiently as a motor at proper operating temp, but running too hot will hurt your performance, as well, no matter what fuel system you're running - that's more a matter of the properties of metal and friction than fuel delivery.
As stated, take into account the rest of your cooling setup and go from there. Even with a 3-core radiator and a big honkin' electric fan, I still need to run a 180* in mine to keep it cool in summer ... but then, I live in a desert climate where temps regularly go above 100* daily (ACTUAL air temp, not heat index, since motors don't know the difference between 100* in AZ and 100* in the Midwest). If your setup tends to run a bit hot in the summer months, I'd say shoot for a 180* failsafe t-stat. Otherwise, stick with a free-flowing or failsafe 195* t-stat.
And hey ... swapping T-stats on a 302 takes less than 30 minutes and a $2 gasket, anyways, so it's not like it's an expensive experiment.
