Double tracking - Do you have the other track running?

I let the first track play along while recording the second. Partially as a direct check to see if I'm not doing something retarded (like the accent thing...I must admit that has happened), and partially just because I find it invigorating. Hearing the song come together while tracking boosts my morale and I like to think that increases the quality of my performance.
 
I track with all previously recorded rhythm tracks playing for consistency in performance, to makes sure all of the tracks are in tune with each other, and to make sure that the tones are working together as a whole. It's hard to maintain the overall picture of the record when you're listening to only two or three instruments at a time.