My mentor in Ann Arbor told me my signatures were too loose, and it fixed my spines
She pressed a finished book flat on the table and the whole block shifted a quarter inch, which was pretty embarrassing. So now I tap every signature down before sewing, but how tight is too tight before the thread starts eating the paper?
Have you tried the wiggle test where you hold the signature by the edges and see if it flops like a wet noodle? I learned the hard way that if you tap too hard you get little dents in the paper right where the thread goes, and those weak spots tear way easier down the road. My sweet spot is tapping till the folds feel solid but the paper still has a little give when you press it. Also worth checking that your thread isn't too thick for the paper, because that eats through stuff way faster than tight tapping ever will.