Managing fuel burn on x-country w/C172

[9 lives]

if it's full, or going to be filled, and standing on any kind of sideways slope where the vent is downhill.

Yes, as long as it is understood that selecting off "both" in that circumstance preserves the fuel in the higher right tank, but you're still going to dribble out quite a lot of left tank fuel, it it was near full.

Taking off a 172 on "both" is the specified practice, and of course should be done. However, understand fuel systems too. The "both" selection is a "best practice", not immediate unsafe condition if not done. A large portion of the GA fleet specifies taking off on the more full tank, even some high wing Cessnas, which do not have a "both" selection. Interestingly (or alarmingly) the way they are placarded requires you to move the fuel selector through "off" to get to the other tank. So don't pause there!