That is a useful strategy, M1k3. It can be lifesaving. I set that for myself a few years back, too, but found that it was a wall, rather than a boundary for me. As I've gotten better with the help of my HP at maintaining boundaries, I can still protect myself while letting myself get to know individuals, whether they have past addiction problems or not.
I've also learned to trust my judgment, and when, like Inpieces said, when I see something that's unacceptable, just go. I can look back at points in relationships in my life when I knew I should have turned on my heel and run, would have saved myself a lot of heart ache.
Take good care