Fillup:
Daily-driving: 1/4 tank, then fill-up at the next convenient time (i.e., within the next day or so, depending on expected driving)
Trip: when the light comes on (if gas station locations are known - 1/4 tank if it's iffy)
Trips (e.g., long, mostly highway):
Before Trip (depends on vehicle) --> see below):
< 2 hrs: Fillup if > 2 hrs: Fillup if 8 tank
This all depends on which vehicle we're taking, too:
Legacy: ~425-490 miles per tank (easily)
Forester: ~300-350 miles per tank (stupid turbo)
I didn't think the extra trip mileage on the Legacy would matter much, but now that I'm spoiled I'll never buy an other family vehicle that gets less than 400 miles/tank highway (at least!)