Sometimes I redose because I want to prolong the high, when I have that many painkillers.
As a chronic pain sufferer though I now don't really have that sort of amount and so I can't really redose at all. I wait until I'm totally sober again. I find it to be much better than redosing. All redosing does is get rid of your pills, really fast.
I will also add my observation, if you are GOING to redose, this information might be of value to you:
*Redosing on Oxy - does little to nothing unless it's at least twice as much as what you took to begin with
*Redosing on Hydro - keeps the fire going, gives you a little kick again at even the same dosage you took before
But that's just my experience. Also might be a tad biased as this is coming from one of the very few on this site who hold the opinion that hydro in equivalent doses is MUCH better than oxy.
Just do whatever works best for you. Honestly, what I usually do now is take my opiates in the morning for the pain (and a little extra, of course) and switch to something else later in the day instead of redosing, like valium.