The 2003 toyota echo has 58,000 miles on it. Runs okay.
I wonder if I should change the plugs, or wait a while longer?

Replace them with the same plugs. This should help:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-5.html#post617111

They're due at 60,000 miles. Read this post on spark plugs and you'll be replacing them before the weekend.

Changing spark plugs (http://www.ricksfreeautorepairadvice.com/repair-topics/spark-plugs-and-wires/time-for-a-tune-up-pick-the-right-spark-plug)

Gas is going up again, <arrg> Squeeze every bit of gas millage you can with that Echo echo echo.
Plug it!