Much is made of how AL teams are at a disadvantage when they play in NL parks because they generally have tell the lumbering Solomon Grundy-types at DH to have a seat. Conventional wisdom states that this is somehow unfair, because they're playing "with less than their full lineup."
The real disadvantage goes to NL teams when they have to play in AL parks. In an NL stadium, they sit down the DH and have 8 hitters and a pitcher in the lineup - just like the NL teams. While I'm sure some slight advantage accrues to NL teams because their pitchers are at least more familiar with which end of the bat to hold, even the best-hitting NL pitchers still, frankly, stink. Or, to put it another way, Carlos Zambrano gets mad props for bashing as many homers as Jack Wilson.
On the other hand, when an NL team goes to an AL park, they need to find a DH, and most NL teams don't have a Big Papi-level masher just sitting on the bench to take the role. If you're lucky, it's your fourth outfielder - the Michael Bourns and Endy Chavezes of the world. Now match the drop in productivity from Hafner/Ortiz/Thome/Thomas to the Olmaedo Saenz/Brad Eldred/Ryan Doumit extravaganzas to the dropoff you get from Zambrano to Andy Pettite, and I think there's a significant tilt one way in that equation. (And yes, a lot of NL teams were using big name hitters as DHs, but that just means they put the lesser lights in the field. The net result on the lineup was the same.)
So the next time you hear a whinge about how Sheff has to sit, and woe be unto the Tigers as a result, bear in mind - Sheff sitting is less of a negative than most NL "designated hitters" hitting.