On: An employee, whose last name is Null, kills our em...

[maxerickson]

Using "if myvar is 0" exposes you to more implementation dependent behavior than "if myvar".

[toyg → maxerickson]

I agree, but it's still more explicit in terms of what process is used to evaluate the content of myvar and what it should match, especially in a context where you're expecting a numeric value rather than a boolean.