IMHO you should always use components vs groups , they give you much more flexibility. The definition of components is invariant when you move, rotate, scale, so one entity can be reused in many cases. The attached model was done that way. At inside corners, as I am sure you know, cope joints would be used since closing miters is hard and miter joints on outside corners. These operations will change the definition ( Su model) although there are ways around that. The attached model does not have any cope or miters so you can see what is going on.
There are a number of forums related to wood working check those out. FineWoodWorking has one Called DesignClickBuild that covers many subjects you may find helpful. ChiefWood Worker also but he has not posted .skirting_mac1.skp (96.1 KB)
Two links for info
http://www.srww.com/blog/
Design. Click. Build - FineWoodworking