1. Shift+Click the building you want to deselect from the selection bar (that thing at the bottom of your screen) then set the control group again.
2. Right before something important (Stims, Siege Mode) is going to finish for you, so that you reach the enemy's base just as it completes. Or, right after an expansion finishes, so you can deny it without him being able to cancel for a refund. Or, whenever you want, because you're Terran. Timings are something you'll only really get the hang of from playing against humans, though.
3. Scanner sweep to check your opponent's tech and decide on what you want to start making from there.