As a rule, when saving seed, you generally do so from the first fruits to mature. This way you are selecting for the earliest. In my experience, those are also the strongest seeds; some of my later tomatoes last year had seed so small as to be non-viable. Seed harvested earlier from the same plants was much larger, and when germination tested, had a much higher germination rate.

Also, if you (or your neighbor) are growing several different tomatoes & you wish to save seed, there is a chance bees will cross-pollinate them. Bagging the first set of blossoms on each plant (before they open) with a light, breathable material (such as floating row cover) will ensure seed purity.

Thanks , just what I needed.
CDG