I try to use units appropriate to the job at hand. If I'm using a Stanley tape measure, I find using English, I can simply read it off the tape. Calipers with both? Finding a drill bit, I use English. Other things, I may well use the units that are easier to remember or are nice numbers. Some math (especially in my head) is easier with the decimal metric, some easier with inches and feet.
Having just one system would be nice but that isn't the world I live in so I might as well take advantage of the best of both.