I downshift. For a few reasons:
1. It helps to slow down with less wear on the brakes.
2. Should I see someone coming up fast behind me, I am in a gear that will allow decent acceleration should the need arise.
3. Most motorcycles are "return shift" and thus you have to go through all of the gears to get to neutral or first anyway and it isn't always easy to downshif 4 times while stopped.
You will often hear the "brakes are a lot cheaper than clutches" used as an excuse to not use engine braking while slowing. While this is a true statement, in 36 years of driving everything from motorcycles to 18 wheelers, I have NEVER worn out a single clutch. My current pickup is a 94 Chevy with 130,000 miles and 17 winters of plowing snow and its clutch works like new. I ran a Jeep Comanche to 150K on the original clutch. Likewise, ran two VW Beetles past 100K and a Chevette to 115K and never wore out a clutch and I always use engine braking.
I have, however, worn out MANY sets of brake shoes and pads, so while a clutch costs more than brakes, if clutches never wear out the argument does not hold.