Yes, this is normal wear, especially on the rear tire. Some people will let the rubber wear all the way down to the cords before they replace the tire (the rubber is completely worn off in areas). Most people will replace the tire sooner than that. Tire performance does degrade when the rubber gets very thin (less grip, less cushioning, more flat tires), so you need to do your own performance vs money analysis. Some tires have wear indicators (grooves in the rubber that disappear as the rubber wears down) to help you make the decision, but these are really just suggestions. Replacing a tire only takes a few minutes, if you buy the new tire before you need it.