I just re-read your question and realized I'd read it incorrectly earlier.
You aren't concerned about another offer, but a counter of your offer if you come in at your top dollar. Right?
With that being said, I'll still stick to my previous answer somewhat, as buying a home is about motivation ... is your motivation such that if you offer your top dollar, and are rejected, you are perfectly willing to continue searching? Or is this the house you absolutely want? If this is the home you absolutely want, then I would suggest being frank with the sellers. Simply offer them your best price, but qualify it with "Mr. and Mrs. FSBO Seller. I would like to offer you such and such a price for your lovely home. This, however, is my highest and final offer, due to financing considerations." Then include with your offer a written pre-approval letter from your lender. This will define you as a serious buyer and perhaps they will see that a bird in the hand may be worth two in the bush.
I'll stick to my other advice about inspections and property disclosures, too!
Let me know how it turns out!