With a bottom up approach you are trying to prove or disprove the model in your head. Or at least only slightly mold it.

There's no way to learn more than that because you've already fixed the foundation. This is a good strategy when you've already been working on something for a while and you have made some firm bets or if you've done this thing before to some relative success.

The sales guys is usually going to be better if you really want to understand a new customer from ground up.

Ie direction matters.