Yes, the regular tennis balls that are actually made for tennis are bad for their teeth. But the tennis balls that Kong makes & other toy companies made for dogs are usually a softer bristle and not so abrasive. Which is why we don't use regular tennis balls.
Another problem with tennis balls is that they collect dirt and grit, which makes them more abrasive when the dog chews on it. Poor Jasper's got quite worn that way. Unfortunately I never found a "dog designed" tennis ball that lasted more than 5 minutes.
I have one long "snake" with multiple squeakers which for some reason nobody has disemboweled yet. The bad part is, we were all playing last night and with two dogs tugging and stomping on it the squeakers drove me nuts!
We picked up two new toys at Petsmart called DOG BAL. They balls that are about 8 inches in diameter made out of kong like material. They have lasted for about three weeks with little damage. This is HUGE. The only problem is that they do have a bit of heft and the dogs love dropping them on the floor till you pick it up an play with them. At times it sounds like a drum corps is marching through my home.