It is a myth -- unless you're talking about cigars that have oils and stuff added to them. A friend of mine who owns a cigar shop once experimented with putting a baggy of black cavendish with some cigars - and they came out smelling like vanilla. Some people like the different aroma / flavor, but I wouldn't experiment like that in your humidor. Maybe put some stuff together in a rubbermaid container.
You are right, though -- a strong cigar and a mild cigar won't blend so long as they're both all tobacco.