|I usually use it right at the very end (unless I am trying to fix up some sky-then I will single out the sky and give it a once over-for my prints), and I will do it at the beginning. Mainly at the end after editing, because you may bring out more artifacts, etc while editing, plus you may smooth out something you don't want to.
I have actually been using it to sharpen my photos also. It opens at 100% and I go the the Filter part (where you can adjust the amounts of sharpening, etc.), and will leave the strength, etc at 10, then move the USM amount up to like 200 and the radius between .4 and .6, then adjust the USM amount to what I like. I will move the strength and smoothness down to preference, also, jus to where the noise I want gone is GONE. I check all over the picture in the little box...find the place I want to make sure looks right, then go from there. Looking around more after.
Sometimes I will just do a sharpening and drop the strength and smoothness down to about 3-4 (leaving contrast up at 10) for just sharpening and smoothing the USM artifact.
I still use regular USM at a very low setting to fine tune after resizing or prior to printing.