The greater the pixel density, the more magnification you have ( im not sure if that's the proper terminology). Have you ever tried live view at normal magnification ? And then look at it at 10x magnification? See how much more it shake it has? So the crop cameras typically have higher density/magnification so therefore you need more precision to take those tack sharp shots, especially at 100% crop.
But the same rule applies if you were to compare the 5D2 vs 5Dc. It's not a crop vs. FF argument.
And especially if you're taking photos with the composition/framing in mind, as my example illustrates, you can take the photo with the 7D/85, or choose the 5D2/135. And you'll still end up with more pixel-on-target with the 5D2 because it's 21mp vs. 18mp. In this scenario, the 5D2 should be more technique sensitive with the 3 extra megapixels to worry about.
I would hope most people take pictures and compose them by what they see in the viewfinder, and not looking saying to one's self, "Well, since I'm FF and plan to crop down to 1.6, I'd have less pixel density to deal with, but I'll deliberately shoot wider and I'll end up with a sharper resulting cropped image."
You frame it as best as you can (and sometimes you're focal length limited) but you do whatever you can to minimize cropping later.
