I can't speak for everyone, but 78" would be steep as hell for me. I run 70" on my ss roadie commuter, but I also have a couple of 1 mile (or longer) climbs each direction on my commute. I might roll 75" on a bike I purpose build to take to the velodrome, but that's about it.

Again, just one guy's opinion. I find 65 - 70 inches to be the right ratio for me.