i would recommend definitely waiting for spring training. My favorite time is around two weeks before opening day, that way people are aware of most of the spring training injuries and the free agents have all signed.
I think you're better off waiting as long as you can before the season starts. That way you not only get to see who is injured, but how question mark players (like Wade Miller) are doing and get somewhat of a handle on any spring training surprises.
I prefer the weekend immediately before opening day, but my league generally drafts a couple weeks earlier.
(In the paleolithic period of '89 I was in an auction league where we held our draft the weekend AFTER the first games ... made for some interesting bids on a few unknowns who jumped out of the gate fast )
Last week before opening day. This way, owners have time to think about trades and sleep on them, if necessary. Before this time is too early...one team every year is eliminated by an injury if you do.
Always the weekend before opening day, allows time for trades but not enough time for interest to wane in managers who have a bad draft (like me, I had all 10 of my guys I want (texeira, Santana, Blalock...) taken the pick before me by a friend of mine)
There are 3 ways to do things, the right way, the wrong way, and the Max Power way!