I just replumbed the master bathroom, including re-routing a bit of the waste/drain lines. After the hot water is used, the pipes make an easily audible creaking, popping sound for about 15 seconds.

Obviously, I think this is due to expansion / contraction with heat, but is it something to worry about?

I'd appreciate any help on this.

Not to worry. While it's bothersome it's not dangerous. I have made up a little explanation. Knocking, popping, creaking and crackling pipes are a common complaint, especially in colder weather. I can tell you what's happening and take the mystery out of it but you're not going to like the repair. When a draw is made on a hot water line the line expands against a pipe strap next to a stud and when you drain hot water from your tub or shower it goes into a chase that is a little cooler then room temperature. The heat expands the pipe causing it to rub against the stud to which it is pipe strapped. This is the sound you hear. As it cools it contracts and the noise is heard again. To repair it you must tear open the walls and locate the pipe strap that's causing the problem and shim it tight. Most people when they learn what causes it just elect to live with it. Regards, Tom