It does rather depend on how accurate you want to be. Millimetres are less prone to reading errors than fractions of an inch. I once had to replace a domed clock glass for a teacher, I forgot to measure it and rang her to measure it for me. There was only a small leeway between it being too big to go in and it being small enough to fall out. It seemed safer to suggest that she did it in millimetres, rather than risk her getting confused by thirty-seconds. Her response was "I can do it in centimetres, if you can convert it".
A lot of die-hard Imperialists are far more metricated than they would be prepared to acknowledge. Photographers have had 50mm lenses almost forever, you won't hear anybody talk of two-inch lenses now, outside specialist circles.
I'm fairly bilingual and use what is suitable at the time. I wouldn't expect to find a metric fence panel. Driving a good deal in Ireland has made me use kilometres rather more than I used to. I had the benefit of living in Cyprus after 1968, so I had three years practice with decimal currency, before coming back here just after the changeover, to find people utterly bemused by it all.
I do find it remarkable that people, often those young enough to have never even bought fuel by the gallon, still think in mpg terms. I did get confused, and somewhat alarmed, by the alleged fuel consumption of the first Getz that I drove, until I discovered that it reported in US gallons. They use Imperial gallons now.
I am, though, still more Fahrenheit in the Summer, but Winter temperature are Celsius now...
