Okay, let me address your question more specifically. I used +6 for my 16" bars. If you put 18" bars on using +8 you can always switch to 16" bars and just tuck the extra wire away. However, my original point still trumps all of what I've written here. It's a lot of work. As I mentioned, I went with 16" but I don't think 18" would have been bad at all. Just not worth all it would take to try it. Good luck whichever way you go.

edit: after some more thought...if you started with 18" and went to 16" your clutch cable and upper brake line (if you're running dual disks) MAY be longer than what you'd want. The internal wiring on the bars would be a non-issue.