Put the UAV where it is most needed.  In some cases it may be with one or two guys who've broken away to cap the back flags or in others it may be with a significant battle elsewhere on the map.  In the first case, if you've just done a scan and no one is back there, don't waste the UAV for a "warm fuzzy feeling", put it elsewhere and then go back to the flag cappers and randomly spot around the flag to pick up any spawners.  Zoom in and tell them what you see.  I put the UAV where I believe it will help the team most.

A tip on UAV placement...many probably know this but here it is anyway...don't automatically center the UAV scan over a flag.  Especially on city maps, position it so that its sweep covers multiple flags as much as possible and DO NOT waste the UAV by having its sweep go over anything but the fringes of the out of bounds.