What's the expectation for retiring due to fuel shortage? Wait until it's gone before heading for the map edge?

Time my exit so it coincides with the last drop of gas burning up? Will I be branded a coward if I leave the board with 2 gas remaining?

And, if I've still got some gas at the end but my opponent doesn't, am I supposed to let him go? I've always figured that if you (or I) get caught gliding toward the edge, you're fair game,

but I've seen at least one other game where a dry-tanked player tried to guilt-trip his opponent into sparing him.