I remember John McEnroe commentating during the USO2005 final when Federer hit a dropshot return against Agassi. JMac said, with a bit of a laugh, it's an unwritten rule that that's not cool.
[Agassi, incidentally, got to the return and lobbed the ball over Fed's head, right to the baseline. After Fed scrambled back and returned the ball to midcourt with a bit of over-the-shoulder improvisation, Andre hit one of the only forehand dropshots I've ever seen him hit, which dropped just inches over the net, out of Fed's reach. That was a great match...
EDIT: Just realized I was thinking of a different point.
But hey, it was still a good one, you can see it at 7:00 in this clip http://www.youtube.com/watch?v=Z2JV97fMtqw