Unless the plow is quite deep, it won't really help you steer the cuts so much as just help you start them in the right place when mortising. That's half the battle, of course.
I have actually just taken a few swipes with the plow to start my mortise when I had the plow set up, that's what I was kind of mentioning - it just makes setting the first strikes of the mortise chisel easier, giving your chisel something to register in, which helps when working with a long chisel. And if you want a clean mortise, makes it a little easier. Not something I'd go out of my way for, but since the plane was there, I did it, and then cleaned the "micro-groove" away with a few swipes of the jack plane after the mortise was done. Kind of an odd-ball way of doing it. Like Sean says, really one normally only does this when working in F&P construction.
" Be willing to make mistakes in your basements, garages, apartments and palaces. I have made many. Your first attempts may be poor. They will not be futile. " - M.S. Bickford, Mouldings In Practice