^^^ this.

If you start posting HHs that begin with "Have no reads...", play fewer tables.
If you review your big pot losses at the end of the session and go "Doh!" alot, play fewer tables.
If you misclick sh!t... play fewer tables.
If you're an hour into your session and don't have a goddamn thing in the notes tabs for a significant portion of the people you're sitting with, play fewer tables.
If you like to chat with the stoned dude from the Netherlands, play fewer tables.
If your win rate goes down but you figure you'll make it up in volume, or rake back, or bonuses or that free sh!t you can get with your FullTilt points, play fewer tables.

And if the main reason you're playing more tables is 'cause you're having a down session and just KNOW you'll get that that monster hand quicker by seeing more sh!t...

well, you get the idea.