The main purpose of Caltrops is to play keep-away, so you want some Recharge in them.
After that, they do good damage over a lengthy time period, so slotting damage and/or procs is not a bad idea, but you rarely get even half the potential damage out of them (the damage is over 45 seconds, I think, so the mob would have to last 20 seconds to get even about half damage). So put extra slots in it only if you have them to spare (I rarely do).