You'll be able to see how the car is put together. All it takes is one read around the S or X forums to read about body panels not lining up, things not functioning properly, etc. Things you'll definitely be able to see on a test drive or in the showroom. If the demo model displays these issues I would have no issue at all declining to configure mine when it becomes available and getting the extension to give them a little more experiance with building them.
I'm comfortable that I'll be able to test drive one and see it in the showroom long before I get mine since I'm an East Coast buyer and around 200,000ish in line.
I would rather have a late 2018 build than an earlier build anyway due to early production run issues that seem to plague all automakers. Could cost the tax credit though.