I'll certainly use the Pandora's standby features whilst I'm using it throughout the day, but when I expect not to be using it for a few hours (incl. overnight), it'll be turned off completely, regardless of power consumption in standby mode. I do the same with my PC, Laptop and every other device.
Kevin_H speaks the truth, however, the Linux implementation isn't going to be that picky. Sure it'll cache data to speed up sd access, but a few seconds of no activity and the data will be written to the card. So unless you have only just written stuff out to the sd card, the write cache should be empty, and thus shutdown should be pretty quick.
That's true on PC and PSP also. If you save in an emulator and then immediately power off, you will lose data. However. if you save, wait a few seconds, and then power off, you will not in most cases. The PSP kinda guarantees this as it will not let you completely power off for a few seconds. The PC can do it in less than a second, so it usually takes you longer to kill the power than it takes Windows to save your data.