This is a comment that I often add to code reviews:

Please use IsNullOrWhiteSpace [instead of IsNullOrEmpty] unless we explicitly allow blank strings.

For some reason, people seem to forget about the white spaces around a string.

Even then, I'd suggest a better alternative: normalize or sanitize input before validations. If trailing white spaces are meaningless then cut them out—be lenient to your users. When copying and pasting, people often highlight extra white space characters accidentally; or just a good old case of fat finger.

Here are a few "safe" sanitization functions I have in the arsenal:

string SafeTrim(string input)
{
  if (input == null) { return input; }
  return input.Trim();
}

string SafeToLower(string input)
{
  if (input == null) { return input; }
  return input.ToLowerInvariant();
}