When developing an application with lots of stored procedures, should you store them in some sort of source versioning system (such as source-safe, TFS, SVN)? If so, why? And is
Absolutely. Positively.
A set of SPs is an interface, that is likely to be modified more frequently than structural changes. And because SPs contain business logic, changes should be stored in version control to track the modifications and adjustments to the logic.
Storing these in version control is a symptom of organizational maturity at a coding level, and is a best practice.