I personally think that anytime in the second half after the running clock has started would be good timing. However, the point made earlier about frequency and conditioning of your starters (to prepare for playoffs) should also be considered and may sway that approach.
I would have no issue with the argument that it should be up to the team on the losing end. When they "give up", the other team can "give up". Not my style, but can at least understand it.