Well, for warriors ArP and Str are the two prime red stats and usually the only thing stacked. ArP has been the favored item until a certain point, I'm not well versed in that point, yet. But for Warriors there's little ambiguity. Everything they do is physical damage when they're dpsing, so they get full value from ArP.
DK's get a little less easy to track, and of course, spec is everything. For Blood DK's you'll get the most value out of ArP as around 80% at least of your damage is physical (DC and diseases will be somewhat prevalent, and possibly DnD and BB depending on the fights, but none of these benefit. Because Necrosis is calculated after the weapon hits, based on them, that actually will benefit from ArP indirectly).
Frost does a lot of physical damage, again, depending on style and fight, but still, there are several key damage sources which will get no value from ArP.
Currently Unholy gets the least value as it relies heavily on spell damage, and its main FU spam is 100% shadow damage (ScS). In 3.3 though they're changing that so that you aren't going to suffer from getting the occasional ArP on your gear.
So, long story short, "stacking" ArP is probably not what you want to do as a DPS DK, but it won't hurt you to get some on your gear either. I imagine Kren's comprehensive compilation of DK DPS wisdom in the DPS forum may have more specific math. =)