When to go DW?
At what gear level/ crit% is it better to have DW over cruelty? Does it ever matter? or is there some magic Crit% I should be at before dropping the points out of Cruelty?
You're generally not going to be getting crit on your tank gear, although I could see swapping in a Mirror of Truth or something for slumming 5 mans. Deep wounds really shines when you can mooch buffs like leader of the pack/rampage, horn of winter/SoE totem, and totem of wrath (or master poisoner/that paladin thing).
Even without any points in cruelty, you should be around 21% crit with most of your specials, or close to 30% in a raid setting with the above buffs. That's where you make up for it.