I ride around the block or whatever to reach 100 km, 100 miles, 200 km, etc., if I'm anywhere within about 10 km of the goal. It just feels more complete to me.

The only one where it is a bit of debate is on the 300K brevet vs double century distance ... or 300 km vs 323 km. Sometimes when I've finished a 300K brevet I've been energetic enough to keep riding to make it a double century ... and some of my 300K brevets have actually been anywhere from 305 to 320 km so that makes it all the more tempting. But if I've had a particularly difficult brevet, I might just call it a day at the end of the brevet and not spend the extra half hour or whatever it is out there to make it a double century.