Obviously you want as many studs as possible, but it all just depends on that particular draft.
If I had 2 RB's already and was looking at Kevin Jones or Andre Johnson, I'd go with Kevin Jones. Now if Chad Johnson were there, it would be different.
Just depends on the flow of the draft, how big your tiers are, etc. I don't think you should pidgeon-hole yourself into a particular strategy. Just see how it unfolds, and make the best decisions from there.
But to answer the question, I would take a stud, even if I didn't need him, rather than take a mediocre player just to fill out my roster. Trade bait is a beautiful thing.
"90% of the game is half mental" - John Madden