Hi Susan,
A tomato is ripe for seed saving when it is ripe to eat. (Some things must be over ripe, eggplant and cucumbers are examples of that.)
It is best to wait until a tomato is fully ripe to save seeds. A bit over ripe is fine.
If really under ripe, you will get no viable seed. If a bit under ripe, you may not get as many good seed, but will still get some that are usable.
It is easiest to think of the fruit as a womb, and waiting for the seeds is like a pregnancy. You want to go full term for the healthiest babies.
Tomato crossing rates are very low. It does happen though. So it is best to save seed from multiple fruit. The chances of all the fruit being crossed is very low so you know you'll have true seed.
Hope this helps,
Remy