EDIT: This question is a duplicate of What is the difference between managed and native resources when disposing? (.NET) and many others. Please answer the others if you have something to add.
According to the Framework Design Guidelines by Krzysztof Cwalina and Brad Abrams, a type that contains instances of disposable types should implement IDisposable.
Is there any other general rule of thumb for when it is best practice to implement IDisposable?
From MSDN: http://msdn.microsoft.com/en-us/library/system.idisposable.aspx
Use the Dispose method of this interface to explicitly release unmanaged resources in conjunction with the garbage collector. The consumer of an object can call this method when the object is no longer needed.
Whenever you alocate resources that must be released, such as files, handles, etc. For example, if you are using Win32 resources (which do not implement IDisposable) you should implement IDisposable to release them.
I normally implement IDisposable every time I need to do a clean up of items. For me this is when writing code that abstracts database/network/filesystems.
It just marks items ready for the Garbage Collector instead of waiting for it to try do it on its own.
Implement IDisposable when you have a class what wraps an unmanaged resource or when your class has a field that implements IDisposable.