With my lawn and my out front mower, I try to keep it within an inch or two. I can see where I'm cutting with that deck out front, but sometimes I drift off a bit and leave a strip unmowed. That ticks me off and I stop, reverse or loop back quick, and clean it up before I continue.
When pulling my rotary cutter cutting brush, grass, and weeds, I try to keep it close but I'll bet it's 6" or 8" of overlap.