I must agree with Terrainsafe, At my station, it's all about the least overall delay. If an aircraft has 3 minutes to go before they get to the apron, I'd push somebody else ahead of him. If they're just about to enter the apron, the push gets delayed. Not an exact science, but I try to keep the guys in the picture as much as possible.
As for ACFT under tow, they'll pretty much have to wait until I can squeeze them in/across with a minimul delay.