It's a while since I've flown the C172 but I recall that if cruising above 5,000 ft the manual advised to fly on either left or right tank. Something to do with vapour lock or some such thing? If you're flying on both and the engine quits you don't know which tank has caused the problem.
The POH for a K that I fly advises to change to left or right tanks after climb. Our 172 syndicate have the idea that it's the fuel vent on the left wing that acts as an air inlet in the cruise, slightly overpressurising the tank compared with the right and causing it to empty faster. Don't know if this is true, might be urban myth. I fly on alternate tanks anyway, a la PA28, 'coz that's what it says to do.