I tend to edit pages only to add United States Customary (USC) units where there are only metric units. If the page is about something in the United States, I make the USC units primary with the metric units parenthetical. If the page is about anything outside of the United States, I make the metric units primary and the USC units parenthetical.
While the United States officially adopted the metric system over 40 years ago, to most of our citizens the metric system is entirely meaningless. Because I spent years in Europe as a child, I'm able to convert most measurements in my head quickly. However, I realize we're just not going to drag Americans into everyday use of the metric system, so adding USC units to Wikipedia pages makes those pages more understandable to Americans. Wikipedia is about knowledge, so anything that makes a page easier to understand is an improvement.