Where I start depends on how bad the undercoat is.
Standard technique would have you brushing with dog on his side, starting and the bottom and systemically moving up. I expect the idea is to create maximum "fluff factor".
But as Danisgoat said, I generally zoom in on what needs getting done. I never do an entire grooming at once - so I tend to spread it around. If their butts are ragged, then I focus on the butt. Because its their thickest part and rarely needs fluffing I tend to work both top and bottom together. My only goal is to get the fur out.
When it comes to belly and ruff, I tend to work bottom up in lines. But not always. Sometimes, I just want the quick fix. Quick fix is easier done top down; as most of the shedding hair is on the top.
I think the more important factor is technique in line brushing, regardless of where you start and finish. The important factor is the technique.
Always brush after wetting the hair with water or leave-in conditioner - never dry.
With your brush, assuming your are right handed, create a "part" . Use your left hand to hold down the fur below and with your right hand brush the fur from the root up with the brush at an angle. Do this in small sections.
To be really effective you want to do this in very small lines, but I normally jump to 1 inch lines, but do it more often.
Hope this helps.