Are you are planning a trip in which you will be using hotel points and changing hotels during your trip? If any of the hotels that you are planning to book are a Starwood, Marriott, Ritz or Hilton hotel, try to stay at one (all?) for at least 5 nights.
I have taken advantage of this benefit on some of our trips. We got the fifth night free when we stayed in Starwood hotels in Costa Rica, London, San Diego, D.C., Scottsdale, Monarch Beach, at many Colorado Starwood ski resorts, Whistler in both the summer and the winter, LA and Paris and Aspen.
However, I have missed it a lot too. Many times I do not get this fifth night free because on certain trips we move hotels too often to stay somewhere for 5 nights. What really hurts is when you miss it by just one night. This happened to us on our trip to Hawaii on Kauai. We missed it by just one night. Ugh!