I just designate approximately. For example, yesterday I smoked a cob that I normally smoke Virginias and VaPers in, but with C&D Pegasus. I burn burleys like that occasionally in that pipe, just because they smoke so well in it. I've never noticed any crossover or anything. A little dark Burley flavor from one of those C&D burleys, though, wouldn't exactly bother me if they came through in a Virginia or something. I do have some pipes that I'm more stringent about, but for the most part, if it seems like a good idea (blend+pipe combo), I just go for it.