It varies for me... depending on the day, the project, the mood I'm in... most often, though, it is one can on and the other ear free.
If I'm doing long narration (like an audiobook) and I want to hear that my voice tone is staying consistent, or my neighbourhood is being slightly more noisy than normal (garbage day, a lawn-mowing or sunny low-flying kind of day) I'll tend to wear them to check for any low-frequency ambient noise interruptions that my mic is picking up, so I can pause until the noise passes. (My studio is heavily treated but I live near a small airport and those little cessnas love to fly low over the town and have a frequency that still just penetrates my 4-layer walls! Those and the harley-riding neighbour down the street!! I can't hear them, but my mic can.)
If, however, I'm doing something that needs a more natural, conversational read, the cans come completely off.