Jarod has a nice post where he demonstrates how to reduce the number of postbacks that occur when using the ServerSideValidationExtender included as part of the Validation Bundle.
Jarod demonstrates how to subclass the PropertyProxyValidator and override the EvaluateIsValid() method to not validate on a full postback. He then shows how to modify the ServerSideValidationExtender so that it tracks the current value for the associated control and only re-runs the validation if the value has changed.
If you are using the Validation bundle, and you are concerned about too many postbacks, then check out his fix. Jarod's even dropped the full source with his fixes applied.