I believe at locations with this setup, the front of the cash register "podiums" all have signs that say "Please use both sides". So, I do.
What can be more annoying are people who don't understand how the Guest flow is supposed to be between the register and the counter. In a facility like CHH or Cosmic Rays, there are three "lanes" between each register podium: two "feeder" lanes and one common "exit" lane. At some locations (CHH and Electric Umbrella are the most notorius, in my experience) after people order, they walk up to the counter through the exit lane. The line of people post-register then forms behind them, up the exit lane, and every single person who exits then has to cut back through the line.
I'm such a geek that if I come across this, I actually try to "self-correct" the problem by hanging back if I can to allow those in front of me to exit, and then I re-form the line at the counter where it's supposed to be.
And while we're on the subject, what's *almost* as annoying are people waiting in the "feeder" lane after the register, but leaning against the railing and into the exit lane making it difficult or impossible to get by carrying a tray full of food...
-Rob