Positional scarcity is clearly important, but you need to be flexible. Don't think of it as having to fill C or 2B by a certain round, but ask yourself which available player will give you the greatest advantage over your competition. If you're drafting in round two and Soriano is the only 2B off the board, the answer may well be 2B: there's a huge gap between Boone and Kent and the next tier of 2Bs, and exploiting that gap will give you an edge. A round later, if Boone and Kent are no longer available (just for the sake of the example), there's no need to pick a 2B any longer. Quite a few available 2B options will give you roughly equal value; in other words, the scarcity has been sucked out of the position for the moment.
And I don't think pitching is too risky for the opening round. If I'm drafting at the bottom of round one and Prior's still there, he's mine.