I try to avoid more than two hitters per team, but I don't mind adding another as the season progresses if I'm just going to ride that hitter's hot streak (like...say Jason Giambi).
As far as pitching goes, I would stay away from having more than one reliever from the same team if you play in a holds league. It's a bummer when Hoffman doesn't get a save opportunity for the week because Linebrink also doesn't get a hold opportunity. Starting pitchers are alright, except for the fact that every once in awhile you'll get two pitchers starting in Coors on back-to-back days and then you are relying on your non-aces to give you quality innings.
Now that's all if you are playing H2H...if you are playing roto, there's not too much risk involved except for the fact that if one of them goes down, it'll affect the other one's rbi opportunities and the like.
For the most part, I try to stay away from stacking up on one team (note: I have stacked up on Oakland A's as this season has progressed, but I only drafted one A).