I would say very. Usually when the valves are loose it creates this rattling rather unpleasant noise but It's not simply a matter of being noisy; the quiet ones are the ones that really have to worry about. Valve train lash that is too loose will beat through the hard coating on the lash adjusters, cam followers, or cam lobes over time. This can be expensive to repair.
Valve train lash that is too tight (the TOO quiet part) can burn the valves, valve seats, and in some cases ruin the cylinder head if the situation is bad enough. Not to mention rough running and lack of performance.
Valve clearances get out of spec due to the valve face seating and dishing in over time, wear on the valve seat, and valve stem stretch, the latter of which is generally true of the exhaust valves. Lack of regular oil changes and wear on the valve train can cause excessive looseness.