Organizations keep official guides for uniforming.  Usually,
if it is not detailed in the uniform  guide, it should not be worn
on the uniform.

The BSA *Insignia Guide* details where to put just about everything
on the BSA uniform for both adults and scouts.  It is very
inexpensive, and available at your local Scout Shop.  Every unit
should purchase at least one  each year, since it is updated on
an annual basis.
An on-line version of the insignia guide can be found at:
http://cochise.uia.net/oldbaldy/formsandpubs.html

According to the Insignia Guide there is only one official uniform for Boy 
Scout
Troops.  It includes the following statement:
"No alteration of, or additions to, the official uniforms, as described 
inthe
official publications, or the rules and regulations covering the wearing of 
the
uniform and the proper combinations thereof on official occations, may be
authorized by any Scouting official or local council, or any local executive
board or committee, except the Executive Board of the Boy Scouts of America
after consideration by the Program Group Committee."

By the way, there are no official class B uniforms, there is only one 
uniform for
Boy Scouts and it is officially called the 'field' uniform, and unofficially 
known
by most of us as the class A uniform.

What is optional:
- Boy Scout neckerchiefs (troop decides by vote)
- Official headgear is optonal, but while in Class A uniform, if you are 
wearing
headgear it must be official headgear.
- The web Boy Scout belt may be replaced with a high adventure base, camp
or Wood Badge belt with official BSA buckles.
- The world crest

For information on the Cub Scout uniform see:
http://www.geocities.com/Yosemite/9152/uniform.html