I usually use inches in the page layout programs I work with, but it's easier to input decimals into measurement fields in the applications, so I have a bunch of the metric to english conversions memorized. It's second nature for me to type .0625 when I want a sixteenth of an inch, or .3125 when I want three sixteenths of an inch. With the work I do in packaging design, we often get dielines created by the engineers with the english units written as fractions (1.25, .6875, etc.).

Of course, most graphic designers are used to juggling all kinds of units, including points, picas, inches, millimeters, and pixels. It's just something you get used to.

__________________
Yes, they laughed at Einstein, but only because of his silly hairstyle; no one was actually laughing at his science.