I think you do. Building up speed on certain things, scales, other peoples tricks, your own stuff, can make you faster, but it wont be in time.

You might rip 6 notes really really fast and then come to a more difficult part and slow down to play that part and you would fall out of time. If you had a metronome you can play it at a certain speed, and the turn it up and play it at that speed smoothly and so on and so forth.