As a (very) frequent "µ" user (and several others like ± and Δ and °) I've had the alt codes memorised for ages but having recently, out of necessity and a bit of a nomad lifestyle, moved to a "tenkeyless" keyboard (the horror), I simply macroed it, but I find it refreshing that some keyboard layouts simply include it. I have an obscure pocket 8-bit with an even more obscure OS written in Sweden that also has µ.

And it's option-m on a Mac.

That's what I do, actually, although I use Alt+230 because I remember the code from CP437.