I've redshirted some seniors before and this can be very effective. If the person is maxed out (all red) then you have little to lose.
If they still have some improvement available, then wait until near the end of the season to redshirt, but if they are maxed out, it doesn't really matter. Even with a 1 WE, their cats wont drop if you put 10 minutest into each category, at least I haven't had that happen. It is the offseason you have to worry about.
During the off season, there is a decent chance that they will lose in several categories (and about -15 or more total) but most times, you will make up most of this loss during their playing senior season. One thing to do is to put more minutes into those cats that they need or are real important for their position (PE,LP, DEF, etc..)
My first NT championship came from redshirting a senior and he contributed greatly to that championship.