When building my Ches LT18, I noticed the keel line had a flat spot in the middle. The line didn't really look "fair". So I loosened the stitches, added in some spacers (scraps of popsicle sticks). Did this until the curve looked good, no lumps or dips, while keeping the gunwale spacing correct. Only had to add an extra 1/8 or 3/16 inch in a few spots (one or at most 2 sticks thickness).

Your spacers do not have to fill the entire gap lengthwise. That will get filled with thickened epoxy when you make the fillets. (Use fairly thick epoxy or put some packing tape along the keel to keep the first batch from dripping out onto the floor. Clean up any drips on the hull with a sharp scraper or single-edge razor blade before it's had a chance to cure completely - it's much easier than sanding it down when dried. Trust me.)

If you're using something like popsicle sticks, let the extra width hang out past the bottom of the hull. It's easier to trim off the excess & sand it down smooth on the outside rather than on the inside.

If your keel line looks "fair" right now, that is a smooth curve bow to stern, don't mess with it. But if something looks a bit off, this is one way to fix it. In general, "if it looks good, it is good" when you're working with a proven design like most kits.

Good luck!