If you notice rough idling or hesitation when you accelerate then you likely have some faulty spark plugs.
You should pull one of the spark plugs out and check to see if it's fouling out or the gap is getting bigger.
If the spark plug is fouling or the gap is getting bigger then it's time for new spark plugs.
Spark plugs usually can be bought for around $20.00 to $30.00 per set so I would replace them if you have any doubt about them being bad.
You'll recoup that money you spent on spark plugs through your gas mileage savings.