before you even worry about % off of the asking price - make sure you do your research in terms of market analysis and comps, and check that the asking price is realistic to begin with.
If the house is being listed 15% above fair market value - even getting 10% off the asking price is not a good deal. Make sure that your offer is in line with market value, and represents a good deal to YOU - not just a good deal compared to the asking price (the seller).
I personally agree with you - and might even consider going lower.
Some people take the mentality that anything less then 95% of the asking price might offend the seller. Well - that's the seller's problem. You have to look at the best deal you can possibly get, and the seller has to look at the best deal he can possibly get.
I'm willing to bet that no matter what you throw out there, you will get a counter. Even if you don't get a counter, and you get a flat out rejection - you can tweak your offer and resubmit it.
I'd say go 90-93% plus your costs, expect a counter - and at the end of the day possibly be able to get exactly what you're looking for.