Stiffer is better, but as the hulls age, they will develop some flex. New boats are quite rigid and you will have to press very hard to get any perceptible flex (this is why racers prefer new boats). It also depends to some degree on where you press on the hull. The H18, for example, has very broad, flat bows. Because of that shape, it is easy to get the bows to deflect.
To answer your question, some degree of flex is generally acceptable, especially with older boats. How much is a little hard to say, but I think you're on the right track. Put some decent weight on the area and if deflection is limited to 1/16" or 1/8" with no crunching noises, you should be good. You can also wrap on the hulls with your knuckles and listen to the pitch of the knocking sound. A crisp, high pitch sound is what you what to hear. A dull thud means the hull structure is breaking down.
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