Fun thread. Speaking as someone who was forced into metrication in 1972 at the age of 12, courtesy of our government's obsession with 'the common market' and all things European, I and most people I know have ended up being pretty much bilingual in measurements, and usually speak a kind of Esperanto down at the timber-yard - e.g. '22 metres of 3x1 planed, two 8x4s of 18mm MDF and half a kilo of inch and a half oval nails...' We seem to get by
FWIW, here in England timber sizes are always given as sawn (sometimes regularised) sizes, so a 4x2 sawn timber is 4x2, but 4x2 PAR (planed all round) ends up being about 95 x 45mm. And an '8x4' here is actually 2440 x 1220mm - sizes are always in metres or mm; ask for something in cm, and they'll look at you like you're from Mars.
Personally, I tend to use whatever's best for the given job - if the centre of a chinmey-breast is 32 1/4", that's what I'll use, but if it's 821mm, then that's it. I have trouble visualising fractional measurements in imperial - 1/16th, 1/8th of an inch - but couldn't go back to imperial screw sizes; drill a 3mm pilot hole for a 5mm screw and you know it's going to be a nice tight fit.
Oh, and as for '2x4' - sorry, that's just plain
wrong
Cheers, Pete.