Wow - some great thoughts on an important subject. I fully agree about not trading a stud after one weak performance, or even a lackluster month. To me, that's not being conservative - it's just common sense.
On the other hand, early in the season can be a great time to trade because other owners may start to panic too soon. Last year, I was able to pick up Tim Hudson on the cheap because his owner was disappointed with his early performance, and there are deals like this available in many leagues. If you have Pedro, by all means hold on to him, but if you don't, it can't hurt to ask if he's available...
I also like the first few weeks for trades that help both teams fill a gap. If you have a lot of speedsters, find a team with Mo Vaughn as its primary sb threat, and see if a power hitter is available. Chicago is right: there's a risk involved in this, since you don't know exactly where you stand, but in categories such as steals, I think you can trust your projections to at least give you a reasonable approximation. Especially if, at some point in the draft, you took the best available player instead of filling a need (as we all do on occasion), this is a good time to convert that player into one of equivalent value who can help your team more.
Good point by uanap about the waiver wire, too. Very often, the wire will be a better source of upgrades early on than trades, even in deep leagues.