It changes from season to season. This season, I'd be fine in the middle to later middle. Just look at "Who would you take after ADP?" thread. Honestly, almost any of those guys would be a viable RB1 - McCoy, Spiller, Rice, Forte, T.Rich, Martin, Foster, etc. As such, you could easily snag one of them at 7 or 8 and come back around after the turn and nail either another quality RB or, depending on roster requirements, go with a stud a WR or even QB.
Several years ago (when Moss and T.O. were studs), I loved later and utilized the studWR/studWR method - and one draft got them both. It was a 10 teamer and I was 10th. Best overall record that year. The other nice thing about that strategy is you are usually starting positional runs from their, not reacting to them. It's one thing when someone snags 1 stud WR or 1 stud QB, but when you snag the top 2...then the next guy thinks "Man, those two are already gone, I better get one..." then at the 3/4 go RB/QB...then the QBs start.
But a lot of it depends on scoring, roster requirements and the talent/situations of that particular season. Some also depends on leaguemates and their drafting tendencies. I know 2 guys in my league that will go RB-RB and just keep adding RBs until their heads explode. Use that to your advantage, wherever your slot it.