Normally if I see that prices are still dropping I wait to fill up. Some people have reported prices going back up. Here, it's been at 2.75 for a few days. Not going up but not dropping either.
So I would say if it's still dropping near you, I'd wait. If it's holding steady or going up I'd fill. It's a gamble but I try not to stress too much. A few cents price difference for a 20 gallon tank amounts to about 50 cents. Then in 10-14 days I get to roll the dice again.
Now with airfare, that's another story! That stresses me out since it can vary by hundreds of $$$.