There seem to be two schools of thought on z-backlash. One is that it is not an issue because gravity takes care of everything. The other, that the z-bearings (even Misumi's for those that upgraded!) require a certain amount of pressure before they slip into motion and that sometimes they might have moved when we rather hoped that they would. All of that I can imagine being sensitive to the parallelism of the z-rods, the lubricant and the temperature. The other day a bloke told me an interesting thing. You can make a leadnut with almost zero backlash by taking a piece of Teflon (eg rods available on aliexpress), drilling it and tapping it for the leadscrew dimensions. Because teflon is spongy (by comparison with metal) it stretches a bit as you tap, then springs back as the tap comes out of the hole. Which means you end up with a threaded hole that will fit the leadscrew really snugly. Moreover, Teflon is self-lubricating. Felt I had to share this in case someone thought it was worth a try.