A bit of both, I think.  The current function is actually 'getvalue' and is responsible for retrieving the value, validating its type, and converting to that type (the current ones do both in the same operation).  It feels to me like a cleaner interface to decouple retrieval and validation/conversion, so that the validation function gets passed a string and returns the desired type.  But in that case, having the string dialog take the validation/coercion function makes the name of the askstring function just wrong.

So, I still think the cleaner API is to expose the class and let the application subclass to provide the validation function.