Zinc coated, grade 8.8, metric socket head:
I am not worried about them breaking in a crash. After a crash I always inspect the bike for damage before proceeding.
My concern is micro movement, vibration, and wear causing one to weken and break WHILE you are riding, the seat collapsing and landing on the rear wheel, causing an instant brake and pitching you off violently...thats all...
It was just a thought.
Even in high grade 8mm factory bolts on my MX bike subframes I have broken them at least 3 times. I speak from personal experience.
I agree with wanting to keep the bolt pocket small, unobtrusive and clean looking. I hate odd frame shapes hitting me in the ankles on a bike, bad designs. An Allen head is the way to go....