I should have made it clearer, you don't always keep your centre of gravity low, just when you see a sweep coming in for your feet. Even if they change their aim and hit your knees, you should be ok. Just sink down then do your counter. Don't be a static target. Judo guys love that.
I've never done randori with a judoka who could knock my legs out by hitting my knee with a sweep or a jujutsu style 'leg plant' once I knew they were going to sweep.
Guys who get you by surprise though.. different story.. watch out for them