There's no magic number, either in the past or today. Offering 20% less may result in a great buy (if accepted), or--if the property's overpriced--may not be a good deal at all. Same with 30%. As other agents will advise, first you need to know the comps. List price means very little; it may or may not be close to the real market value. Good agents will set the list price at or slightly below the comps, but there are a lot of other agents out there, and a lot of owners looking for yesterday's higher prices. So, don't focus on the list price. Focus on comps--the recently solds.
Second, regarding "insulting the owner," get over it. So what? You come in low, and the owner has 3 choices: (1) accept, (2) counter, or (3) reject. If he accepts, fine. (Except that means you could have gotten it for less.) If he counters, fine; that means you're in the ballpark. And if he rejects, then you move on.
You're not competing for Miss Congeniality. You're trying to buy a house. Base your offer on the comps, and then, if you wish, go below those.