I hear that getting too much oxygen in your wert before the boil can cause "hot side aeration." I used to let my runnings from my MLT just fall into my kettle, then I heard about this, so I bought a 2 dollar tube to hook up to my spigot.
Anyway, I've yet to notice a difference in my beers. I have a couple that are 4 months old that I made before I got the tube and they still taste great. In fact, they taste better everyday. Maybe it only matters in beers you are aging for a year plus? Thoughts?