"...not worth his salt"

Where did the phrase referring to "a man not being worth his salt" originate?

I don't have any reference to hand but here's my understanding:

Until relatively recently, salt was a very precious commodity. Roman soldiers were given money specifically to buy salt or were paid with salt, thus the word 'salary,' from the l*t*n 'salarium.' Salt became equated with wages, so a person not worth his salt did not give value for money.