It's all about the bees. If you don't bag blossoms, then heavy bee activity in your yard makes the isolation distance thing moot (how fast can a bee get from one side of a 50 foot garden to the other!). And with tomatoes it is not about wind pollination - it is about insects.
So, I plant my tomatoes 3 feet apart in rows 4 feet apart, but save seed either early (before heavy bee activity) or late (more risky, but after bee activity subsides). My success rate has been excellent.
And - no, potato leaf varieties are no more apt to cross. It is just easier to notice, since it is a recessive trait. I grow lots of potato leaf varieties and my seed purity has been 95% or so. Any heirloom beefsteak variety has stigmas that extend beyond the anther cone, as well as the occasional very large marigold type blossom that is likely more attractive to pollinators.
So, to be on the absolute safe side, bag. Or, if you know your bees, don't.
The other point, and this is key, is that tomato flowers being "perfect" (both male and female parts), the vast majority pollinate upon opening - so even bee visitation would not cause problems if it is already a done deal.
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Craig
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