I rarely know any troops who backpack and hike in Class A (or field uniform or whatever they're called now) so it would usually be Class B. B's are generally a scout related shirt. The bottom half is up to the wearer.

In Crews (I used to be a crew president, adviser and Ranger) the uniform is up to the Crew except in situations where the Class A (green shirt, grey slacks) would be warranted.

In Sea Scouts, kilts are a no go, as in any other loose fitting clothing and necklaces.