you know, i was thinking about this too, but i really dont worry about it anymore.
the vita is a frikkin beast, i leave it in sleep mode for weeks, it only completely goes off when i forgot to charge it for weeks , sometimes i do not play it for a couple of weeks and i leave it in standby mode, when it is fully charged and i leave it in sleep mode , i can leave it like that for a few weeks and turn it back on and still have nearly 60-70% of battery life left.
i love my vita , i love my n3dsxl too but i never ever leave my 3ds'es in sleep mode because they die so fast :/
maximum is a day or two for its standby time with nearly 50-80% of battery remaining.
whereas with my vita having 50% , so half of its battery bar left ,i can leave it in sleep mode for over a week easily.
long story short, it is fine to leave it in sleep mode just try to not leave it in sleep mode while you have a game running (though even this is fine xD to my surprise i once forgot to exit out of a game and left it laying , it went to sleep mode and i forgot about that it was still in game and i left it like that for a couple of days and it did not even affect the battery life).
but to be on the safe site just make sure you are not in a game, i also run taihenkaku and that good stuff.
so to not be required to re-run the browser/email to get into taihenkaku mode again, since i am so lazy i always keep my vita in sleep mode.
i hope we get coldboot cfw for the vita, that would be fantastic, and i would then even consider turning my vita off completely every few days to give it better rest i assume.