A few months ago I advised caution when picking DE, since BoP blues often vendor for more than shards sell on the AH. Lately I've started watching greens as well.
Dust has crashed to near 1g each on my server. Sometimes higher, sometimes lower. Either way, it's not much. Low LK gear will only give 1-3 dust and very low chances of a cosmic essence; meaning an average of 2-3g from DE. Cloth vendors for little enough that it is still profitable to DE. But plate I've often seen at a nearly 50% loss. Mail is a loss as well. Leather is mixed, but I'd pick DE to save the bag space and vendor time.
As you auto-run to the next pack, take a second to glance at the vendor price. If it's 4-5g, it's probably not worth vendoring.