To the best of my knowledge, it does not make a difference whether you sign in on initial setup or afterward. The difficulties OP was describing, I think, are tied to activating the "Find my Mac" option in iCloud. Once you activate that, you cannot simply log out from iCloud without providing your password. Being able to do so would rather defeat the purpose of "Find my Mac" as a theft prevention mechanism.

So if you don't want the issues described, don't opt into "Find my Mac" (Personally, I'm quite fond of the option, but YMMV).