There does seem to be something in it. I just did some research of some heavy derby defeats I particularly recall. I appreciate that it's still a very small sample.
Anyway:
Spurs 0-5 Arsenal Dec '78 - (6 games for next Spurs win)
Spurs 5-0 Arsenal Apr '83 - (1 game for next Arsenal win)
Spurs...
After Brighton 1-3 Palace in Sept 2011 (7 games before a Brighton win)
After Palace 3-0 Brighton in Dec 2012 (5 games before a Brighton win
After Brighton 3-0 Palace in Mar 2012 (5 games or more before a Palace win)
Just goes to show that derby day thrashings are probably worth more than a...