I like to turn up the card from the bottom in my "major" readings--that is, the twelve-plus-card readings. I usually lay out all the cards then turn up the bottom card as the finale, placed just below and to the side of the layout. It's the card signifying, to me, what's underneath the entire reading, the unconscious, or as some folk have put it, "shadow" influence. And because spreads with that many cards can sometimes be all over the map, the underlying card often proves very useful. It can bind the entire reading together, to give me a hint about what it's *really* all about. For example, if the card is, say "Justice" then I know that whatever else I read in the spread, the real issue here is about how the person can balance out their life. This can help in interpeting the other cards and how they relate to each other.
On the other hand, I don't usually pay much attention to the bottom card if I'm only doing a short 3-7 card reading. In those cases the top cards usually do the job just fine.