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I'm just about to the point of assembling the the neck and installing it. Fretboard is cut to shape and inlay is done. Up till now, I hadn't paid much attention to how the neck fit into the dovetail, but now that I'm to that place, I see that the top plane of the neck, which should be co-planar with the soundboard, is about 3/16" lower.

It takes 4 shims, nearly 1/8" total, on the sides of the dovetail to bring the neck up to flush. How important or unimportant is this? I figured that it would be somewhat critical that the bottom of the dovetail should bear solidly on the bottom rather than be wedged in place with shims.

Any recommendations on how I should handle this?

I think OP needs some in-person guidance from a real luthier. A bunch of interrelated elements need to be resolved in sync with each other and swapping notes is a very hard way to comprehensively learn. And each question will get four answers, two of which will each uniquely contradict the remaining two.

I suggest that at the least OP contact the vendor of his kit and see if there's a path to knowledge. OP's asking about lutherie, not woodworking.

Best thing is that OP realizes he's in over his head and has raised an alarm.

if the neck is tight you are fine
the trick is gluing them in. Take some wax paper cut a few strips.
glue the shims in and use the neck tenon as your clamp. Don't over glue
clamp tite and let set about 5 min and check the shims
wipe out excess and reclamp if they look good
post some pics we can tell you more

There's nothing wrong with shims. As long as the end result is a tight fitting dovetail, you're good to go.

...what banjopicks said. I suppose there must be people who can lay out and cut a perfectly-fitting dovetail at precisely the correct angle... but I'm not one of them.

Regarding shims, I've tried every material in the shop and was making shims as needed out of mahogany or maple. I think it was Diane who posted somewhere in here that she uses maple veneer for shims. It cuts with scissors, is consistently flat at less than .02", and comes in sheets. 15 bucks got me enough for years.

https://www.ebay.com/itm/Maple-Wood-Ven ... 2749.l2649

I've cut some shim stock in a range of thicknesses and am still tweaking the dovetail to get a good fit of heel to body. I thought I had a perfect neck alignment, but noticed I still had a teeny gap at the heel. Every time I adjust the dovetail for a tight fit at the heel alignment by "flossing", the neck drops a tiny bit and I need to use a thicker shim.

A lot of trial and error, but I'm getting there.

Red Dogg wrote: ↑Tue Mar 23, 2021 7:29 pm I've cut some shim stock in a range of thicknesses and am still tweaking the dovetail to get a good fit of heel to body. I thought I had a perfect neck alignment, but noticed I still had a teeny gap at the heel. Every time I adjust the dovetail for a tight fit at the heel alignment by "flossing", the neck drops a tiny bit and I need to use a thicker shim.

A lot of trial and error, but I'm getting there.

The neck "drop" is a common problem with first guitars. Trying to get the neck to sit squarely on the shoulders is daunting. I think I know what's happening.

When you scribe sand, you have to be very careful about how you pull the paper. If you pull straight out, you may be inadvertently pulling a tiny bit upwards. Pull downwards. I find where the neck is holding me up and just run the paper back and forth in that spot sometimes, without pulling it through. Use chalk to find the problem area if it's not immediately noticeable.

I have built 9 with an M+T joint, and done only one dovetail re-set. It seems that the only place the dovetail has to fit really well is the bottom 1/3 to 1/2 because that is the part of the joint that is tension. The top of the joint is in compression so not necessary to be tight because the heel cheeks take the force. Same idea using one bolt instead of 2, that bolt being at the bottom of the joint.

While it is good workmanship to get the whole thing to make good contact, the only really important part is the lower part.

Does this make sense?