What I've heard more talked about is balance.
Ie, a 1Ds with a 50/1.8 attached feels too unbalanced, as does a 1100D with a 70-200/2.8L.
For the first, there's nowhere to put your left hand, you can't rest the lens on it, best you can do is hold the left side of the camera body.
For the second, I've heard a lot of reviews saying "adding a grip will make a long lens more balanced with a lighter body, and reduce hand/arm strain". The theory is that a heavy lens will produce more torque around your wrist, if you add a grip you're adding more downwards force, but it ends up as less torque rotating the lens around the axis point in your hand.
My 70-300L and 7D go nicely together, but sometimes I can definitely feel that extra torque rotating my right wrist around. I haven't got a battery grip to add to it, but i've heard from others that it makes a lot of difference to wrist-strain at the end of the day...
As for how that affects camera shake and image blur, longer focal length of course means you're going to need faster shutter speeds. It also depends on how strong you are, before you start getting the shakes from handholding too long. But a longer focal length (and thus heavier lens) on a really light body like the 1000D is going to put more strain on your left than right hand, balancing it out with a grip will mean that you're taking more weight (as a % of total weight) and shake will probably be less.