Invoice deals won't be common until dealers have a hard time selling at MSRP. At this point MSRP is still a great deal and those trucks move quickly. I suspect it will be a couple of years before demand dries up and dealers have to price these trucks aggressively to move them. And if Ford keeps boosting prices $1-2k per year, you probably won't save that much by waiting.
Toward the end of this year you might be able to get better pricing on a leftover 2017, but I doubt many dealers will discount 2018 orders. Way too many buyers willing to pay MSRP at this point.
Demand doesn't have to dry up, just slow down. That's what happens when the initial craze dies down, and the novelty of being the first guy on the block with one wears off. Already many dealers are willing to move at MSRP without a lot of negotiation.
Even last year there were reports of discounts from MSRP, but those deals were pretty few and far between and seemed like insider deals. At the time, MSRP deals were few and far between, but now people are getting them all over the country.
Fact is, it's expensive for a dealer to keep a $60k or $70k truck on the lot. And the other big fact is that dealer holdback on a $60k or $70k truck is big, so even moving a truck at invoice is going to be a solid sale for a dealer.
A year ago and up to a few months ago, dealers were scrambling to get any allocation they could, with demands exceeding allocations. Soon, allocations are going to exceed demand (arguably already happened), and the smart dealers are going to be selling trucks and taking their profit instead of letting allocations go unfilled waiting for above sticker or full sticker.