I don't think you want to build wash-in into the wing
This will take from the stability of the craft by encouraging a tip-stall before a nose-stall
I doubt it's very serious here in the SSE build, otherwise the instructions wouldn't be so casual about it, but I can't see how it's advantageous
I'm guessing that it's neglected in the instructions so as to speed up the build
What you're shooting for here is absolute symmetry in the wings - added wash-in will result in a plane that is somewhat more stable inverted and somewhat less stable right-side-up
What you can do is mimic the profile of the tabs by using a tapered strip of balsa...or the above suggested method of carefully re-attaching the tabs
The other thing to watch out for is that your root ribs are nice and square and true...watch out for mild warps in the root rib ply - this can be countered with judicious use of water ie. dampen the side that you want to make more "convex"

I.F.

I build, therefore I am