I've done it, though it isn't a habit so much as a practising thing. I certainly sing when driving (I almost never have passengers) as UK radio seldom has any music I want to hear, and the speech station can get a bit heavy.

Tangledwood's post made me giggle and recall walking through Leeds a few years ago with the crowd of other office drudges when a tall, gangly, bloke in red velvet appeared and started turning a succession of cartwheels downhill against the flow of suits: he got LOTS of space, but he had a big grin, and even caused a few little ones.