Depends on the cards, and depends on why I want them, generally. When trading for something like rookies to finish a set, then I like to try to keep things even as far as BV goes, because they have value for me when I complete the set. BV works well in this instance, because it is usually pretty simple to make these kinds of trades. In fact, I'd probably make similar trades without the BV, but it just gives me something to check on.
For my player collections, and other shiny things that catch my eye, I go based on the way I value the cards, which is quite different from BV.
This distinction might only make sense in my head, but that's usually the way I think when I am looking to make trades.