In general the stalemate timer almost never becomes a factor in the US format because our games can last upwards of an hour (due to the 2 30 minute halves).
I think the last time you commonly heard about people timing out rounds was when we had cp_ashville in the rotation - and I think all of us can agree that was the problem more so than the timer.
Sometimes if a team goes *really* tanky on a last (heavy, pyro, engee) and you fail push hard a few times the timer will get down into the 3 minute range, but then desperation will usually set in lol.
With the presumption of a 3 minute time, and the running of uber, you'll only get 3 shots at pushing last at best, and there's also absolutely NO incentive to push out, because pushing out to second is much harder than defending last 1 more time, not to mention if you harass the attacking team with a sniper (thus delaying the push for every pick you get). You could easily delay each push over one minute and limit them to 2 pushes in 3 minutes. Since you've gone all in defense it wouldn't be hard to keep them out and just reset the round to a new mid fight - which I think would be bad for the game in general :/
The issue with pushing out of last on most maps is:
1. The distance most teams put between themselves and the doorways into last, if you aren't holding close to the doors you simply can't push back to 2/4 as well, because by the time you mop up everybody and get to pushing they're respawning or respawned by the time you get to second because of the amount of distance you've forced yourself to have to cover.
2. Heavy/engineer off classes, that guy you had off classing, in general, not only has to participate in mopping up players, but then has to run back into spawn and then cover the length of the distance from spawn to 2/4, which again, makes you slow (often too slow) to 2/4