I am a TechEd veteran. The scheduling tool is, as Mike pointed, used to help distribute the sessions to an appropriately sized room, though, during on the presenter, you can figure which ones will have a big turnout. Regardless, schedule for all of the sessions that you interested in. I usually make a couple of passes before the conference (note that more sessions may be added anytime before the conference, so definitely check the schedule before heading to NO). I pick everything that is of interest initially and then cut it down to no more than four sessions per time block. I then print out my list and keep it handy, though you should verify the rooms as these can change. This way, I go to my first choice and it's full, I can quickly pick my next option.
Hope this helps....Chris