Yes! A late plane to some markets (ie, NYC, BOS, DC, etc) can costs much more in service failures than a few more mins of gas, though, we are always looking for ways to save gas. FedEx for the most part tries to get the planes to push back and taxi in some sort of priority, but sometimes delays such can change that order. I always thought ATC was in constant contact with FedEx working to get aircraft order shifted in necessary!