I understand that virtual methods allow a derived class to override methods inherited from a base class. When is it appropriate/inappropriate to use virtual methods? It's not always known whether or not a class will be sub classed. Should everything be made virtual, just "in case?" Or will that cause significant overhead?
When you design a class you should have a pretty good idea as to whether it represents an interface (in which case you mark the appropriate overrideable methods and destructor virtual) OR it's intended to be used as-is, possibly composing or composed with other objects.
In other words your intent for the class should be your guide. Making everything virtual is often overkill and sometimes misleading regarding which methods are intended to support runtime polymorphism.