Using the wrong version will result in a "Version Mismatch" error as soon as you try to launch the game. You can always find the latest builds on the official SKSE website . Why version 1.6.640 matters

: To prevent Steam from automatically updating your game to a newer version and breaking your SKSE setup, set Skyrim's properties in Steam to "Only update this game when I launch it" and always launch via the SKSE loader.

Given that Bethesda has released newer updates ( 1.6.659 and 1.6.1170 ), why would anyone deliberately stay on 1.6.640 ?