After you have this great name for your dynamic IP, you should still consider it "dynamic". This means the IP number can change and this influences the "quality" of the name.

You don't want to run your production, high-profile webserver on this. Suppose you lose the IP due to a glitch and the IP gets reassigned to the webserver of the kid down on the block sharing his "interesting images" collection.

You really don't want to point your MX records to dynamic names. When the IP gets reassigned and you don't change the A record (because for example your machine decides to hang during the day and you can't fix it until the evening) people will get bounces on mail telling them about "relaying denied" (hopefully) or the mail vanishes into a black hole.