Good rule.
These types of trades always kill me. I saw their guidance was well below expectations and I was able to put out a short quick enough to see a nice profit, I was just too slow to cover. When the stock bounced because of the positive reaction to IBM's number I should have covered immediately, but it is hard to buy back at break even when I had a nice profit two minutes earlier.
Any time I become hesitant and stubborn when trading after hours I usually get burned. On the flip side, it is this volatility and immediate gratification that draws me to after hours.