As I've said pertaining to finding mint errors in Mint Sewn Bags: (same concept applies, in general, to proof sets and mint sets) "You can go thru 1,0000 mint sewn bags of XXX, and find absolutely nothing. You can then go thru the 1,001st bag, and find double strikes, off centers, and off metals" IE; you just don't know, but the odds are against finding anything significant in any group of mint sewn bags, or proof or mint sets - not counting minor struck thrus, etc. is slim or none.
The only instance where I have relative success with Mint bags of coins is looking for high grades, but it's up to the individual to know how to grade. Chris