I've just always seen this as a stereotype almost in every mafia related media where they say they don't want to deal drugs, but end up doing so. I've seen examples of both mafia leaders refusing to deal morphine and heroin as well as those who willing got into the business. As a overall which would you say was more prominent?
Not very true at all. Check out The Five Families by Selwyn Raab for an in-depth look. While there was talk of a Mafia-wide ban on narcotics trafficking, in general the families couldn't reach a consensus on the matter. Some banned trafficking nearly entirely (Chicago), most paid lip service to the notion of a ban but didn't raise the issue unless law enforcement was involved.