If I am arriving after 9-10 p.m. to a Ritz or a Marriott resort, do I really need to notify them? As a plat, am I not guaranteed my reservation either way?

And, do I stand more chance of an upgrade if they give away my standard room/fill up and we show up at night? (On the other hand, they may give away a suite available for upgrade to a gold or something.)

I guess what I am asking is which strategy is better to increase chances of a nice upgrade?