How accurate does corner angle measurement on crown moulding need to be?
Cannot really answer your question because everything depends on you no matter what tool you use. Here are the four main factors:


How crafty you with the tool when taking the measurement. Holding, sighting and marking are all part of this.
What type of saw and saw fixturing you use to make the molding cuts.
How you handle and use the saw tool.
And to a lesser degree the type of molding materal used.