Yes! Pre-filter aggressively! And, yes, sometimes client relationships just aren't going to be as remunerative as you hope they'll be.

The question I put to you is, would you rather get good at extracting nickels and dimes from clients of a wide spectrum of quality, or would you rather get good at making sure the clients in your pipeline are mostly all high-quality? Billing for scoping time is a good way to get good at the former, and a good way to stay bad at the latter.