Now that I have some, I can’t imagine doing a build without PTFE tape on-hand. It’s good for so many things, including multiple parts of stabs. ( Edit; I used some today. )
Another essential, which at the moment isn’t the easiest to find, would be “North-facing compatibility washers” - essentially ring shims of a very specific size and thickness that are fantastic not only for keycap compatibility on upside-down switches, but very useful for dialing-in stabs. This is especially true in builds that feature reduced-travel switches, which are more popular than ever. The shims can help balance a stubborn space bar in general, but also prevent it from teetering on a long pole, for example. (I also used these in the post I linked in the first paragraph.)