This may sound crazy to you, but aren't forums meant to be a place for discussions?

I have applied for three work permits, and NEVER mentioned my UCI in application forms, and I never had any issues with it.
I also never told someone to disclose rejections or lie in application forms, mate.
Having said all that, filling out the UCI field or not is not gonna have an affect on the application process. And what's
waiting an addition day or two in an application process taking 12 months? Not a whole lot if you ask me.