If we don’t check the EventArgs in a protected event handler, FxCop raises CA1062:
Validate arguments of public methods
Yes, someone could theoretically inherit from my page class and call my protected event handling method as though it were a standard function. Dude, I don’t care.
Suppress it?
Many FxCop rules are suggestions, not hard rules. If you disagree in a specific case, suppress it specifically. If you disagree generally, disable the rule.
That being said, I wouldn’t suppress CA1062. There is a really easy fix, that doesn’t touch the public interface of your method.