If there's anything left, I'll serve the last couple few pints out of it. I have a kegmenter with a floating dip tube, it's spunded so generally carbed, and cold crashed in the fermentation fridge. Generally not too hard to finish up that little bit before the next batch needs to go in.
It goes down the driveway as I clean out my conical. I put enough into the fermenter to get at least my target amount out. I fill one keg (either a 2.5 or 3 gallon keg) and then can the rest. I have an old ale in process (started Saturday) that I might bottle instead of canning. Depends on how many bottles it will fill after filling the keg. If I run out of bottles, but still have beer left to package, it will go into cans as well.
I ferment in stainless, so I never really know how much is left after filling kegs. If I prepare a container, it's usually to big for what is left, if I don't and plan on dumping it, there seems to be more than I expected, so I have just gotten in the habit of dumping it.