When is it appropriate to use virtual methods?

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?

First a slightly pedantic remark - in C++ standardese we call them member functions, not methods, though the two terms are equivalent.

I see two reasons NOT to make a member function virtual.

• "YAGNI" - "You Ain't Gonna Need It". If you are not sure a class will be derived from, assume it won't be and don't make member functions virtual. Nothing says "don't derive from me" like a non-virtual destructor by the way (edit: In C++11 and up, you have the final keyword] which is even better). It's also about intent. If it's not your intent to use the class polymorphically, don't make anything virtual. If you arbitrarily make members virtual you are inviting abuses of the Liskov Substitution Principle and those classes of bugs are painful to track down and solve.
• Performance / memory footprint. A class that has no virtual member functions does not require a VTable (virtual table, used to redirect polymorphic calls through a base class pointer) and thus (potentially) takes up less space in memory. Also, a straight member function call is (potentially) faster than a virtual member function call. Don't prematurely pessimize your class by pre-emptively making member functions virtual.

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.