The problem with deciding to cancel the upgrade request 24 hours before (as a PLT at least) is that if I am auto upgraded my exit row seat is gone. Sometimes they won't even give you back a coach middle seat if they are over sold.
The other side of the coin, waiting to put in the upgrade request 24 hours before reduces your chances for it to clear on a trans con red eye.
Personally, I pass on the 757s red eyes (before the new seats came in) but it is worth it in next gen Biz class seats.
Is there any way to look at the seat maps of the same flight a few days earlier to see if you get the next gen Biz class seats. I got them on a MIA to EWR run mid day and was pleasantly surprised! ^