The real question do I tear off a couple hours work and redo with 1/2 to 3/4in overhang?
NOTE: I did a test pouring water on the shingles to see how it shed - it shot off the top of the shingle and never even contacted the starter strip. But as the shingles wear and their surface oil content lessens, the water will wet the shingles, stick to them, and the who knows where capillary attraction might take it.
I do know for a fact that on a very low slope commercial roof (but not flat), the water will wrap around the drip edge lip and run down the vertical portion onto the fascia if it is touching.
If the slope is high enough, zero overhang is not a problem - but what is that slope?