Not necessarily. Tomato flowers are "perfect" . . . & "closed" . . . or, somewhat so. (Not all varieties, I've been told.)
My dad's youngest brother gave me seeds to my grandmother's tomato about 20 years ago. I didn't even think about them crossing for the first 10 years I grew that tomato - in with a number of other varieties.
I really should be more careful with that one but, as best as I can tell, it still has exactly the same fruit on exactly the same plants as when Uncle Marv gave them to me back in the '80's. I do have saved seed from multiple years, multiple plants.
This year, I've got something over 20 varieties in my tomato patch. With the unusual ones, it may be only 1 or 2 plants. I save seed from the heirlooms. I figure what's the worst that can happen? But, my experience has been that welcoming back the same ones is almost a given.