I've often been able to use one of the standard sizes in place of the
less common metric sizes such as 18mm.

Yep, same here. I have an imperial socket that fits the nut on my VW
steering wheel. I'm sure it must be a metric size, but I inherited the
imperial socket from my step-dad and it works fine. So I've never bothered
tracking down the metric socket.

Sometimes a nut will be deformed just slightly so that the actual metric
socket doesn't fit, but an imperial size will. Six point sockets are much
better for this sort of thing than 12 point sockets.

Anthony Watson
www.watsondiy.com
www.mountainsoftware.com