Yeah, if you can get a .com, grab up the .net and .org too. But if it's not a big deal, get the other one too and hold it. I had to settle for a .net, but grabbed the .org too. The .com is being hoarded by someone. Anything since then, I try to get all three even if I don't know if I will use it. (If it's worthwhile.)
Hmmm, now a slightly different question as well. If someone picks up the .net, is there a chance thats helping your .com site? If they are promoting, probably a lot of people will try the .com instead of .net, which is free traffic :-). You wouldn't loose anything, since I can not imagine anyone typing .net instead of .com.
Maybe a thought not to register .net when you take the .com?