for a long arm I would recommend having a plug in the ceiling so that it can be plugged in above (ditto a light bar if you get that) in a way that is keeps it toward the back of the machine and out of the way. Mine plugs into the wall at the back--since I rarely do pantos from the back of the machine, it works ok, but inovlves cord dragging along behind the machine.
for your regular machines I'd say cabinet height so you don't have to crawl under a table to plug/unplug.