Thanks for your posting, Frankee. I don't necessarily understand the physics behind a string "rebounding" after it has been tuned down. But it does seem to happen. It's very slight, of course, but then again it really doesn't take much of a change in tension to throw a string out of tune.

Another helpful hint (along with my earlier one about tuning UP to pitch) is to first loosen the string to bring it close to the desired pitch, then pull the string outward gently to stretch it before fine-tuning. This can be done very quickly and unobtrusively -- it makes retuning a lot quicker, in fact, than trying to fine-tune without stretching the string ("hang on, I've almost got it"). I find that it does help prevent the string from rebounding in the middle of a tune.