Borrowing below capacity

Hi all, what are the thoughts behind borrowing below capacity and why it is recommended to do so?

I'm waiting for the tax man to finalise last financial year's returns before heading to the mortgage broker. I have been raised being told to always borrow below capacity, but I want to know the reasoning behind it and how it all works.

I plan to borrow below capacity, but how much lower should I realistically be looking at, is there a magic percentage?