Is "cutting in" necessary when painting vinyl siding?
You obviously have to coat the bottom face of each lap. How you do that is at your discretion, but full-size rollers don't make it easy. You'd need a big, sloppy paint load to get good coverage, and this leads to dripping and splatter. 

I've traditionally used a brush for the undersides. Do that first, then roll over the brush marks on the vertical face. Don't work in direct sunshine if you can help it, since latex dries quickly and you won't get the brush marks rolled out well. 

There's a plethora of edger devices out there. Some may be worth a try. There are no rules about this sort of thing, so use what works for you.