Putty knife : when to use flexible blade and when to use stiff blade?
Flexible "putty knives" find more application when performing operations such as doing drywall taping. Particularly when larger width blades are used such as 6", 8" or 10" sizes. 

Smaller sized blades in the 1" to maybe 2.5" sized are more often used with the stiffer blade for operation such as spackling over small areas. 

I have found it invaluable to keep a stiff blade putty knife of the following style available. These come with a relatively sharp edge which makes them very useful for scraping down high bumps and removing paint edges that may be left when something like a door stop or base show molding is removed. The flat end nose of this tool also works very well to open spackle and putty can covers.