Not really, the best thing you can do is change your computer power settings. Like make your display turn off after a certain amount of time and also inactive drives. An idle GPU doesn't really consume that much. Underclocking 2D mode can lead to instabilities as well if you go to far.
Just set a power plan. Its not recommended to underclock, it may simply cause instability, an idle gts 250 wont consume much, you can also enable EIST in bios, it decreases voltage, and makes the cpu run at a lower multiplier, thus saving power-
i do that with my 8800gts, underclock it to the minimum in msi afterburner and set that as a profile, then switch to overclock profile when gaming. It would save power, how much i dont know, and it also drops temps by a couple degrees at idle.