What many investors do--and you should consider doing, if your lender permits it--is have multiple preapproval letters at different amounts. For instance, Joe Smith might actually be approved up to $700,000. But he will also have letters stating that he's approved at $650,000, $600,000, and $550,000. So if a property is listed at $595,000 and he makes an offer at $550,000, he doesn't have to send over the one revealing that he's qualified all the way up to $700,000.
In fact, Elvis is on target--no problem if the pre-approval falls a bit short. That'll really help in negotiations. And, of course, Larry's right, too: the listing price doesn't matter; what matters is your offer and how the prequalification letter lines up with your offer.
I hadn't heard of Chris' suggestion to block out the actual amount. I'm sure she knows what works where she is, but in other areas of the country, such a letter would be met with skepticism. To use the example above, if you're offering $550,000, the seller might be dubious; after all, the preapproval letter might only say you're qualified up to $450,000. I don't see that as strengthening your offer...which, after all, is the whole point of the prequal letter. Still, ask your Realtor what the custom is in your area. If that's accepted there, then that might be the best solution.
Hope that helps.