The neck angle on those ES-1 kits can be very shallow, so you don't want anything to make it shallower. You definitely want the neck to sit like this, but you need to check that you don't need to try and increase the angle more:
Those neck tenons often seem to be curved for some reason. Maybe they finish them off on a spindle sander, which is a stupid thing to do. You certainly want it flat, so you can clamp it securely without risking the neck angle changing.
I'd stick one or two pieces of a hard veneer (like maple) on the bottom of the curved tenon area, then sand away the excess so the whole bottom of the neck heel is flat.
Before doing that, with the neck clamped in place as per the photo above, first level the neck using the truss rod, then check with a straight edge as to where the edge will hit the bridge when set to it's lowest value (wrap tape around the bridge posts so they will fit in the post holes, and allow a bit more height for the raised edges of the post inserts). If you are lucky, the straight edge will end up pretty much sitting on top of the saddles or up to a couple of mm higher. If unlucky, the straight edge will sit below the top of the saddles. The more below, the worse it is.
Test it out, and take a picture if unsure.
With strings on, the bridge will need to come up by a couple of mm from the 'straight edge just touching the top of the saddles' position. Too much neck angle and the bridge will sit very high and not be that stable. Too little, and even with the bridge sitting on the body, you can end up with a high string action you can't do anything about. It needs to be that Goldilocks 'just right' angle. Better to find out now than after you've glued the neck in place.