Yes. On EasyJet (A319) it happens at below 80 passengers or so (i.e., about half full) - I forget the actual number but I've been on a few flights where this happened.
Easyjet doesn't block rows off. Individual crew members may block certain rows off to achieve an even spread through out and below a certain pax figure (considerably higher than 80 pax) a zonal headcount is completed.