After watching many Let's Plays and noticing how much thought I put into organization of my own hotbar, I've come to the realization of just how important hotbar organization is in Minecraft. And yet, I've never seen it discussed before.
Personally, I go with this arrangement:
1. Bow or, if possible, item enchanted with good knockback
5. Solid, placeable blcok (usually cobblestone)
7. Gravity-affected block (sand or gravel)