It can help. It depends on how careful you want to be. Select tomatoes that bloom at different times. Plant one variety earlier than another. Or just plant one variety each year. Or you can bag the blossoms since tomatoes pollinate themselves. By planting other plants in between, you will entice the bees and other insects to go there, but there are some bees that will spot your heirloom past the corn and think that the pollen is sweeter on the other side of the corn.
I remember that seed growers would find a location where they are miles from other crops that might cross pollinate.
We can only do the best we can and then leave the rest to God. It would be a good idea to carefully label seed and plants so that you know what seed you are dealing with and the results. Always keep seed from multiple tomatoes to insure that when one is cross pollinated you have others that are not. Of course you want to plant seed from more than one tomato since you won't know until the tomatoes are produced.
It is a good time to learn about such things so when we need the food, we won't be surprised by a total crop failure.