As an investor, you don't ever have to short a stock.
Shorting brings with it unlimited risk and is very difficult to do.
But I'll tell you what you should do, and that is listen to what established short sellers are telling you. I guarantee that by listening to these guys you will avoid some mistakes.
In the audio link below there is a current interview with Jim Chanos (Trades, Portfolio) who is perhaps the best know short seller of our era.
The interview is very lengthy and well worth listening to.
http://media.bloomberg.com/bb/avfile/Masters_in_Business/v30hE74rTjd8.mp3
