Properly reassembling a machine before returning it to a customer is an important step of the repair process.
This is a list of things to do it right, and things not to do wrong.
One common mistake we see when people try to mess with their macbooks is called the longscrew. Some machines have screws that are meant to be screwed in a metal hole that is soldered on the PCB. The PCB has no hole in it however, so the screw needs to be the right length. If it is too long, as it is being screwed, it will dig into the PCB, and potentially damage some tracks, possibly in the internal layers if it gets deep. So make sure you never put a screw that's too long in these kind of holes.
Generally speaking, don't assume that all screws inside the machine are the same, pay a lot of attention to what you unscrew and what goes where so you can properly reassemble things the same way.
TO DO: Pictures of each machine type with description of which screw type goes where
I find that the ifixit guides for logic board replacement provide good documentation of all screw lengths for logic board to top case mounting. Just as an interim resource until someone else possibly does some specifically for this Wiki. I usually Google something like “ifixit+logic board+Macbook model size type and year. For example ifixit logic board MacBook Pro 13” retina 2015” yields this as the best result https://www.ifixit.com/Guide/MacBook+Pro+13-Inch+Retina+Display+Early+2015+Logic+Board+Replacement/45134