The problem as I see it is that as soon as you affix hard & fast rules to any operation you start to alienate people who manage quite successfully to do that operation in contravention of those hard & fast rules.
Experience counts for a great deal & the ability to assess risks & minimise them is what experience is all about, the problem is gaining that experience usually means making mistakes along the way.
I've always worked on the theory that if it feels wrong then it most likely is & if I'm not happy doing something then I won't, pointless taking chances.
