Concerned? well, yes and no... you need to find out what is causing it (it will only get worse) however it may take a while, these suckers are tricky sometimes. It shouldn't hurt things too much, however your gas mileage will have likely dropped.
You may wish to double check the spark plugs to make sure none are cracked ... I had new plugs/wires installed and developed a "chuggle" 13k miles later... turned out the #6 plug was cracked from being installed a bit roughly. Come to think of it... it was put in the #3 spot for the time being...and it'* still there!