Ideally, you'd want to be ready to hold off at ANY position and go for value where you see fit. Now the world is not ideal and usually I find it best to evaluate position runs and decide whether you want to participate. The only position I'd say NEVER hold off on would be RP. My reasoning is simple: You want to have at least 3-4 solid RP's when you leave the draft and ideally 1-2 prospects at the position as well.
Many will tell you to wait it out and play for the high-reward Derrick Turnbow types but you can do BOTH.
Look at it this way: Every year this time a guy is running round talking about how he "won saves" by waiting till the twelth round to select his first closer and then he ended up with Street, Jones and C.Cordero and therefore there is no value in going in early.
Conversely, the guy in second maybe drafted Rivera, Hoffman, Batista and Kolb and dealt Kolb for a front line starter to a desperate team in early May.
as long as a guy's closing you can deal him so I say load up on them and PARTICIPATE in the runs, even if you're guy loses his job and is a disaster you can just as easily write that off as typical "roster churn" just like the guy who spent a mid-rounder on Pavano.
Otherwise, Plans B and C should be addressed in advance so that you cn wait as long as you can draft completely from a position of power.
And I consider drafing many relievers throughout the draft as drafting from a position of power. Why? Because when the guy next to you sees your staff loaded with Rivera and two Corderos the heat will be ON HIM to scrape up answers for his own roster.