If the game follows the iLevel patterns of the past for combat gear, the first L90 crafted gear available from Dawntrail will be iL645 gear, and the L90 Poetics gear will be iL660. (Of course, the L90 poetics gear won't be available until reaching the end of Dawntrail's MSQ.)

Top crafted gear now is 640, but it can be upgraded to 650.
And the top tome gear for casuals is 650 as well, which I believe can be upgraded to 660 by grinding Myths of the Realm.

Do people think it's worth the effort to grind for 650 or perhaps better gear in prep for Dawntrail, or will the 640 gear be enough?

Having 6 characters currently at End-of-Content all in 640 gear for at least three classes (SGE-WAR-DNC), I don't want to put a lot of effort into grinding gear upgrades if it isn't useful. And I expect I'll easily be able to gather and craft mid-expansion gear (e.g. L96 gear) should it be useful to do so.