Funny you should ask, I was just researching this myself a bit:
This is for my own house, buy I'm a wood guy, not a stone guy. I have an existing brick chimmney on my house which is about 19 years old, the crown looks like crap, and the top 3 courses of brick are shot. I had a mason friend of mine give me an estimate to repair the crown and cover the chimney in cultured stone. What I was pretty surpised about was the lack of knowledge regarding the manufacturers recommendations regarding clearance to the roof and flashing at the top. I have to dicuss with him more, but he wasn't exactly filling me with confidence.