While poker is an endless game, time is a relevant factor. I think the main value of setting stop-losses (like 4 BIs in a day) is that it is built-in tilt protection, regardless of whether you are playing bad or running bad.
As for quitting while ahead, this is a real psychological problem which must be overcome if you want to become a big winner. Definitely set stop-losses, but let your profits run. If you are winning, keep playing as long as you are playing well. The desire to "log a winning session" or a "winning day" is really about fear of losing.
These are all artificial frames that we employ to trick ourselves into feeling good or bad about the situation. More important are rules to not play when tired, etc. Know yourself well enough to set limits on losses, min and max hours for a session. This structure and discipline will make it easier to forget about the fluctuations in bankroll.
http://en.wikipedia.org/wiki/Prospect_theory