Husky roller guide, properly modified for Stihl chain, of course.
Why? It holds the cutter so I don't have to and is excellent for dragging the chain around the bar; and it's excellent for getting me back on track after a few hand-jobs.
Kind of funny when you say hand jobs, ha ha. If you run a saw all the time you shouldnt need a guide, but when I was younger I liked the 4in1, that cheap piece of steel that had angles, and a raker gauge. Now I just use it to clean the bar.
E-lux roller guide, in .325 Pixel on Oregon/husky/Jred chain (95VP), and 3/8" standard on 7XLG, 7XLP, Stihl RM and Stihl RSC (slightly modified on Stihl chain).
They help me hold the file high enough, otherwise as Glens stated.
I remove material on the inside just a smidgen so the links don't bunch up and I raise the rear with plastic zip-ties looped around the rear top of the frame (in the grooves) because the Stihl chain cutter recedes at a shallower angle than does the Oregon the guide is designed for. The groove depths are slightly adjusted as well to achieve the correct overall height.