How much gap is allowed between the door and striker plate
The two primary factors are latch operation and weather sealing (where applicable). 1/4" (6mm) is getting fairly large, but still within what I'd consider acceptable. It's hard to say without seeing it all in action.

Much more than that and I'd take your approach of shimming the hinges, but use some nylon or other non-biodegradable material in the case of an exterior door. 

You could also surface-mount the strike plate, but this does look a bit less professional.

A less significant factor is the appearance of a large gap. Generally, as long as the gaps are individually uniform (parallel), they can be what they are. Varying gaps are what look sloppy.