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

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

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 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