I'm curious what everyone's answers are going to be but I think that
@Pangaearocks has been going down to about Level 1500 using time saving techniques, and then proceeding to carpet bomb the snot out of everything ASAP. Someone recently posted a graph that indicates that the rates of Iridium Nodes increases at a quick pace to Level 100 and then proceeds more slowly after then, so for some practice runs, it seems like banging down 100 Staircases or so is a good start and then start the Bombing.
Going on a high Luck day plus eating and drinking a Luck buff each increases the chances of Shafts once you start Bombing and I think also increase the chance of Treasure Chest floors which can contain very nice things (and also Parsnip seeds...).
As you get better at Staircasing, go for Level 200 or higher before starting the Bombing. I don't know how much day is ideal to spend bombing so I really hope that
@FilthyGorilla or
@Pangaearocks chime in with the time of day they stop descending and start the real mayhem.
I'll answer for me if only to provide a noob's strategy and hope that the more experienced can suggest better things:
Do you wipe out floors that have too many mobs? Or just staircase down another floor?
I staircase down as monsters are a pain, unless the floor coverage looks nice and dense.
Do you also bypass floors that don't have many nodes?
Yes.
Once I get down far enough to start clearing, should I open every crate? Do they sometimes have prismatics or other useful stuff?
I don't as the times I've been tempted, nothing of value has dropped.