I’ll cut to the chase. The answer is never, unless you're a big wimp with no backbone, which, I must say, many writers are starting to sound like with all this queryfail and agentfail nonsense.
Anyhoo, check out this great post by Toni McGee Causey over at Muderati. It’s a moving read. The link comes courtesy of agent Janet Reid, who I learn from daily and will constantly adore, no matter how many people keep saying nasty things about her.