Seriously, who cares if the seller is "offended"?
Besides, unless you're the Amazing Kreskin, there's no way of knowing.
To address the second point first, some sellers will be offended if you offer a penny less than the asking price. They already think their house is worth more than the listing price. Especially owners who've in some way customized or spent a lot of time on the house. I once talked to a seller who had a 3 bed/1.5 bath duplex in a blue-collar community. He wanted maybe 25% more than the comps. Why? Because he'd spent months--literally, about 3 months--meticulously painting each room. He was convinced that this justified an unreasonably high price. On the other hand, I've spoken to investors who've bought properties for 30%-40% under what the sellers were asking.
In short, unless you're a mindreader, you have no idea whether an offer will be considered "insulting."
Now to address my first point: Who cares?
A seller has three choices when receiving an offer: (1) Accept, (2) Reject, or (3) Counter. If the seller accepts your first offer (even if it's only 80% of the listing price), you probably offered too much. If the seller rejects the offer, you offered too little. You could come back and offer more. Or you could wait and--if the property is overpriced--wait for the seller to reduce the price. If the seller counters, then you're in the ballpark. You're in the game. And you haven't given up too much at the beginning.
And the seller isn't going to be your lifelong friend. Nor is the seller going to write you a glowing recommendation for your job search or for your small business. If the seller's "insulted," so be it.
And finally: At least you made an offer. Let the seller be insulted with all the people who took a look at the home and then didn't even make an offer.
Oh, and a P.S.: Make sure your offer is no greater than the value of the property. And never end up paying more. In the example I gave above of the seller with the duplex, you could have offered him 80% of what he wanted and still have overpaid. Always, always, always start off by know what the property is worth (based on comps). Then, if you choose, offer less. Don't assume that the listing price is an accurate reflection of the property's value.
Hope that helps.