Heinous means bad. That's the actual definition, and for all of their strange ways of speaking, Bill and Ted do tend to follow proper rules of grammar.
Non-heinous is therefore good.
Non-non-heinous is the opposite of that, and is therefore bad.
Non-non-non-heinous is therefore good, although I don't think they ever use a triple negative.
Non-non-non-non-heinous is therefore bad. The extra "nons" aren't just for emphasis; each one properly reverses the effect of what follows. That's a quadruple negative, so it's a positive state. "Positive" in this case doesn't mean "good" -- it means you don't reverse it. The state of heinousness has been flipped back and forth a total of four times, so you're back where you started, with something bad.
Bill and Ted tend to use "non-heinous" for good and "non-non-heinous" for bad. I don't remember them ever just saying "heinous" for something bad. It's usually "non-non-heinous". Again, if "non-heinous" is good, then "non-non-heinous" must be bad. And that's how they use the terms. I only remember the one quadruple, but it's definitely for something bad.
Edit: wow, that was a triple in the clip you included. I completley forgot that one.