Rule of thumb that I follow for assigning access rights:
Assign users to groups, assign access rights to groups at the folder level.
BO allows you to assign rights to a specific report for a specific user, but that becomes a nightmare to maintain. You want to assign rights at the highest level possible. So, if you group reports by folders and then assign access to the folder instead of to individual reports, you've greatly simplified your security model.
When you assign rights by user group, not only does it simplify your security model even further, it also makes it MUCH simpler to say "give user B the same access as user A." I even have one or two user groups that only have a single user who needs special access to something so that we can follow this model.
-Dell
A computer only does what you actually told it to do - not what you thought you told it to do.