It all **really depends on** your usage of stored procedures and business requirements.

There are a number of projects that do use a three-tier architecture and depending on the nature of business requirements there might be need to shift some operations to a **data tier.**

Speaking about terminology, in general words these tiers described as:

- **The presentation tier**, or user services layer - gives a user access to the application.
- **The middle tier**, or business services layer - consists of business and data rules.  
- **The data tier**, or data services layer - interacts with persistent data usually stored in a database or in permanent storage.

In usual case, for the given architecture, **the middle tier** or business services layer, consists of business and data rules. However, sometimes it makes big difference to shift heavy set base operations and/or data rules to be done in **data tier** - through set of stored procedures.

Thus, it is really a case-base approach which has trade-offs in itself. However, Microsoft design guidelines of [Three-Tier Architecture Model][1] recommends to keep your **business logic** in middle-tier.


  [1]: http://msdn.microsoft.com/en-us/library/windows/desktop/ms685068%28v=vs.85%29.aspx