Good question, and I am not sure...
I would think that most women who use tampons don't change at every visit to the WC and that if the string does not get dirty that it shouldn't be a problem...But we are not most women...and we are dealing with different types of output then they are.
I guess that you know your pouch and body best and that if you are worried then don't take the risk. I didn't do it systematically but only whenever I felt that it was important.
Sharon