I've heard that it's best to put it to sleep instead of turning it off, as booting up puts a lot of stress on your computer. When leave my MBP for an extended period of time (when i sleep etc.) I usually sleep it. There was a discussion on this thread about this type of thing. One user said that he/she routinely leaves their laptop on for ~1 month straight without any noticable slowdown. Personally, I put it to sleep.