There's two lines of thought here
1. If he comes on as a sub, he's rested as fatigue sets in on others.
2. If he comes on as a sub, nobody really knows how much he can give.
I think that I would start him and see how far he can go. If you sub him in, you're pretty much counting on him being there for the rest of the match - extended period and all. If you start him, you can follow his progress and maximize substitutions. Plus, starting him will take some pressure off Dempsey in the front and hopefully allow for better possessions.
Klinsmann has to ask him if he can go 45. If not, depending on the situation, he's got to wait until late in the game - like around the 80th minute. If he can't go at all, it's nice mind games on Klinsmann's part.