You should...at the inputs to your system. Check the data when it enters your control. After it enters your code it's almost always possible to statically ensur

You should...at the inputs to your system. Check the data when it enters your control. After it enters your code it's almost always possible to statically ensure that the data meets your invariants (hopefully they're well documented). Thus, programming defensively at this point doesn't do anything but slow the system down by adding redundant checks.