I'd carefully peeled back/remove the shingles to identify the the size of my repair (I am assuming it is a repair). The last time I repaired a roof was just last month with a break in weather. I peeled back the shingles and exposed the rotten sheathing around two vents, got in the attic and drilled a small hole next the the trusses, got back on the roof and blazed through it with my PS300 flush to the trusses. Scabbed on some 2x4 to the trusses and nailed on the sheathing. Size of the repair was just at 6'x 4'. I considered plunge cutting with my skilsaw but the jigsaw made quick and accurate work of it all.
I'm not a pro-roofer, but patience made for an excellent repair. Unless I was in a hurry/emergency, I would definitely peel back the composite shingles before cutting.
My 2 cents, Brian