Most thread strength guide lined claim that after 4 threads of engagement the "strength" increase with each additional thread of further engagement results in less and lass increase, of "strength". Or in other words, 5 threads should be enough given good thread forms/profiles.

The anti rotate washers are not really needed but I still do like to use them. More than a few times I've used simple flat washers in place of the serrated anti rotate ones. I do make sure the caliper's mounting bolt and nut are properly tightened. If the bike has a "classic" external cup headset take care of caliper/cup interferences to insure the caliper sits flat, which helps insure proper tightening ability too, besides the avoidance of friction when steering. Andy