Validation at the presentation layer makes for a better user experience.
Validation at the 'lower' levels is important because the presentation layer may not be what is modifying the data. Maybe it's a bulk import program or maybe it's SQL Studio that is modifying the data.
The most important validation is at the database level. Because if invalid data gets in the system, all hope is lost. And maybe someone's job!
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It is never wrong to perform also some validations (like required fields, numeric fields, ...) in the GUI.
http://shoutingwords.com/creating-3-tier-layered-application-using-c-sharp.html