It depends on what you are planning on installing. Since I was planning on air conditioning, I could not get past the tail feathers without getting the A/C kit, which needed to be built into the tailcone. I also wanted to pre-install my strobe and power supply into the tail, so those things got ordered. Oh, and the ELT, so I could do the unit and antenna mounts. And also the built-in oxygen bottle. Then there's the pitch servo for the autopilot, the static kit, and so on. For the wings, I needed the strobes, trim and roll servos, pitot kits (heat and Safeair connections) and lights. When you move to the fuselage, you'll want all your antennae, etc.; I needed my AHRS to fabricate a mount, my ignition control boxes and VP200 boxes to locate those and fabricate mounts and so on. The entire process of building a kit-plane becomes a materials management problem and you have to balance between having things on-hand and potentially sitting around unused until you need them with having to wait until something arrives to proceed.
I tend to err on the side of having things sit around, if possible, but notice that I haven't ordered my engine yet; there's nothing in the construction that requires it until the airframe is pretty complete. On the other hand, I've had my TruTrak servos for a couple of years now. If TT changes them, I won't worry because the ones I have will work just fine. Trying to install only the latest and greatest can be a zero-sum game sometimes; what if you order your servos just before you are ready to install and, while they are in transit, a new model comes out? The ones that arrive didn't suddenly become useless just because a new model came out. Chasing a newer version is fine if it doesn't cost you or hold up your timeline but otherwise let it go and work with what you have. That's why my RV-6A is still chugging alone on the BMA EFIS; it works - even though BMA went under and other products are now much nicer.
