I'm sure there are custom rifles, both highly decorated and expensive and others not so blingy and expensive that don't shoot well. The fact of the matter is that a big majority of them left the gunsmith exhibiting pristine accuracy and had it until it got cleaned in the wrong manner or not cleaned at all until accuracy was gone.
There are no guarantees in life, but there are a lot of people willing to play the accuracy roulette game, whether its for a $500 rifle or for a $20K+ rifle. A good custom smith capable of building and asking a huge sum of money for a custom rifle won't let it leave his shop without knowing that it is highly accurate. However like other professions, there are probably some custom smiths who can't create the complete package, and they may make good money for awhile, but the word gets out quickly.
As an aside, if being into group speak is how you get loved here, I'm probably not going to get a box of candy on Valentines Day from anyone here, but I can live with that. In the world I live in calling someone a jerk isn't a normal thing. I guess for others it is. Se la vie.