That last statement answered your question. You will not see any diffrence, really. Becuase you will NEVER have 100% even wear between all four tires, changing just teh one tire at that point is fine. Now, chaning a tire with the other three at say, 3/16" thread left, not so good.
What I have usually done is, prior to replacing the one bad tire, I measured the tread depth of each tire, found out which corner was reciving the most wear, and put the new tire there.
I have found in ALL of my rides that the passenger side front tire is always wearing about 1/8" faster than the others, probably do to many more right hand turns and what not.
