They're decent and durable trannies but it's generally adviseable to flush the ATF on occasion to help keep it lasting a long time. I've seen a few earlier 850 trannies die between 140K to 200K miles with its original ATF (which is amazing considering) but flushing the ATF with clean fluid can only help keep it from dieing a premature death.
At 70K miles, you should do a 70K mile service (timing belt), flush the ATF if not done before or looks dirty (dark brown) and it's advised to replace your coolant t-stat as well to prevent it from sticking in the near future (fairly common).