The rule is in all the codexes. Generally, I believe, under the heading 'Using The Army List'.
I hadn't thought of checking that, but upon looking in my
IG codex under that section and all the other "non fluff" sections at the back, I couldn't find it. Either way,
WYSIWYG has never been an issue at any of the tourneys I've been to (and certainly not at any friendly game), and I use the
OP's logic on many of my vehicles. That is, they have basically every little bit and gubbin crammed on somewhere, and I just let my opponent know what it's got and show him my list. Even worse now that armored treads are gone...