The adjustment can go out in 2 ways, tight and loose. If it becomes loose you will hear the clatter and being a little loose is not a real big issue, mostly caused by cam wear,(not very likely) it just causes a little more noise and may hamper performance a little, you maybe not even notice.
Being tight on the other hand is more common, and is caused by valve seat wear or the valve seating better after being used for some time. If it's too tight, the valve will not seat long enough to cool properly and then you end up with a burnt valve, and maybe a burnt valve seat and that is a problem,,,,, lots more than $400.00,,, and will adversley effect performance, enjoyment, and fuel mileage, and it doesn't get better over time.