I use a 5 blade razor. I have nicked my shin once or twice, but never my knees.
I don’t know if hair all grows at the same rate, but the stubble on my knees seems to grow much faster. There’s nothing quite so unpleasant as having the sharp spikes of stubble sticking through 15 denier hosiery when you run your hand over your lovely smooth legs! Or the lacy hem of your nightie “catching” on them as you swish around the kitchen in the morning.
So I shave my knees, without fail, everyday. And my upper thighs so my hold-ups hold up. The lower leg can go for a few days.
Fortunately my 40denier opaque’s are much better at suppressing them.
On the subject of shaving I have found that dry shaving my legs in bright light is much more efficient. You can see the hairs better. And the razor doesn’t glide over the lubricant you have coated them with! Works well on the arms too.