It's not wasting time, since you're doing something useful. Wasting time is when you do absolutely nothing (like sitting in your base). Because expansions are so important, creeping it is almost never a waste of time.
Once you get your expansion up, portaling to it is definitely a good idea. If your opponent runs, you can hope to get a few of his units while he runs (especially if you have Dryads or Shadow Strike). You lose Gold, he loses it too (he may even portal out himself). Just buy another Town Portal scroll and continue the game.