I agree that nails can be rebalanced time after time, but it depends on how great the work is done by the tech. Because if the nails are always lifting, and you start to get fill lines, then it will look bad as the nail grows out. Especially if the client is coming to you from who knows where!
I had a lady come to me, and she had fill lines from the cuticle all the way the tip of the nail (gross), it was like the rings inside of a tree. And the acrylic was extremly stained a yellowish-orange (i assume because they didn't use base coat and this woman loved to wear dark reds). I immediately soaked her off and gave her a new set! She didn't even think their was anything wrong with them!!
Anyway, it also depends on the product as well. Some acrylics start to yellow, or look dull, (if they do, use something else!) and therefore I believe it's time that a new set should be applied, for a fresh, new look.