As Hemera said, I think it depends. (And I agree with the idea of it possibly being partly due to auto suggestion.)
I constructed a crystal grid to promote creativity. It's rarely within my auric field, yet it seems to be working. In theory, a grid is more powerful because its energies are amplified both by the stones being linked to each other and by using them together with sacred geometry, all of which could make it easier for those energies to effect things at a greater distance.
On the other hand, the packet of crystals I have to promote sleep only seems effective once I hold them for a minute and ask them for help. Though even then, they're still not directly in skin contact with me, they're in a pouch. And they continue to work through the night while under a pillow. My labradorite palmstone works amazingly when held, reasonably well when in a pocket, and only weakly when out of arm's reach. It's very good at appearing in my mind's eye when I need to go find it, though.
So for me, it's something of a tossup.