If you plant more one variety there is always a chance of cross pollination.

Just to clarify, in case this is your worry:

Cross-pollination is only a problem if you're trying to save seed. Cross-pollination will not affect size, shape, color, or flavor of the fruit. The pollen-parent's genetic contribution appears only in the embryos of the seeds of the new fruit, not in the tomato flesh (or even the seed coats).

If you're worried about saving pure seed to grow next year, see the FAQ on preventing cross-pollination:
http://faq.gardenweb.com/faq/lists/tomato/2005025852004159.html
You can find the little tulle bags at Walmart in the wedding aisle of the party supply section. I believe you have your choice of white or silver.

And the larger question to me is for what purpose are you saving seed from some OP varieties.

If for home use then you need not be so concerned but if trading seeds or SSE listing them, etc., then seed purity is more of a concern.

I'm glad you were directed to the FAQ here on How To PRevent Cross POllination b'c there were several of us that spent a good deal of time on that and then it was approved by those who were here at GW at the time. And I'm not all that biased but I do thonk it's perhaps the best discussion of X pollination on the net, and with pictures to boot.

Different insects that can pollinate can fly in excess of two miles, so there's that.

My rows were 250 ft long, plants 3-4 ft apart within the rows and rows 5 ft apart and my cross pollination rate was about 5% meaning that of seed saved from 100 varieties that on average 5 of them would be cross pollinated.

There are lots of variables that play into X pollination and there's an excellent article at SOuthern Exposure Seed Exchange written by Dr. Jeff McCormack who owned SASE before he sold it and he outlined in great detail all those variables.

So while my experience where I grow my stuff says 5% there are places where it can be up to about 50%, but that would be rare.

Carolyn