This question was asked some time ago on another post. Enigma's answer was that the ratings are in fact for everything - sinker as well as bait. So a 7oz rating would mean a 6oz sinker plus a medium-sized bait, more or less.
But one's casting style has a lot to do with it - guys who REALLY load a rod with a powerful pendulum technique or such could snap a rod, whereas a gentler / simple over-the-shoulder cast would not with the same weight on...
I've heard of some guys with gentler style get away with exceeding specs by a long way too, eg. an 8oz sinker on a rod only rated to 6 (not a chance I would want to take though).
Personally, I generally cast with a "mild" pendulum action and not a massive drop - max 2m. On my heavier rod which has a 5-7oz rating I have cast a 7oz sinker plus a bait that weighs at least 1oz, sometimes more, many times with no issues whatsoever.
If I am wrong please correct me, but that's my understanding.