When you add extract you'll want to stir long enough to incorporate it into the water. If you don't it will pool on the bottom and can scald or caramelize. Beyond that there isn't much risk once it gets going. I tend to stir the hell out of mine when using a big outdoor propane burner just because the surface of the kettle gets so hot.
Stirring is a simple operatiion. If something would get stuck to the bottom of the boil pot, and then scorch, a whole brew can be lost. Keeps the hops in the boiling wort also.
The hop sacks do tend to sink after they get saturated well. i like to stir my wort just enough to keep it moving on it's own. Keeps the extract from scalding on the bottom,even when thouroughly mixed,ime.