When I see a post with exclusively US measurement units (inches, feet, miles, gallons, ...), is it appropriate to add (in parenthesis) converted metric values?
I'd say it may not be worth an edit on it's own, but if you saw other things to edit as well then go for it. On the other hand, I myself have made insignificant edits (title changes, removing "thanks", etc.).
If you feel like contributing to the community through edits, it's not the worst thing in the world. So, why not? Just be mindful, folks don't like it much when there are bulk edits. Maybe only do a couple at a time, so the main page is not flooded with your edits. Or try to keep the editing to the weekends, since people tend to be more accepting of mass editing on weekends.
It might not be worth it to edit every measurement, but if you feel a question/answer would be improved by the edit I don't see a problem with it.
Also keep in mind, editing for the sole purpose of obtaining badges is frowned upon.
There are scenarios where it is usually useful. Temperature is one of them I can think of - giving the conversion allows someone who is not familiar with the other measurement to understand the question properly.
For dimensional measurements, it depends. Some measurements are only really significant because they are relative to other sizes. For example, 16" OC studs and 8' long drywall because the edge of the drywall will be at a stud. In this case, giving the measurement in metric isn't useful because I don't think there is a standard that says "space your studs 40.64cm apart". However with something like "dig a 3x3ft hole 6in deep", converting that could be useful so that I could dig a similar hole using metric measurements.