I've just read a book on gunsmithing by a gunsmith who actually replaces hex-head screws with slot heads wherever he finds them. The problem with hex heads are as follows:

1) Poor quality steel in bolt head
2) Poor dimensional tolerances in bolt head
3) Poor dimensional tolerances in hex key
4) Wrong allen key used in hole

Any of the above could lead to the edges being taken off the hex hole meaning the screw has to be drilled out. Drilling out is a very precise job requiring a special jig to get the precise alignment. It was enough to put me off hex heads anyway.