Cross pollination has come up in a couple of threads so I thought I would dig out some pictures.
Note the size of the bees (smaller than a house fly). They only stay at a flower for a couple of seconds at most so unless there are a lot of them working the plants they are very hard to notice.
A tomato self pollinates in a couple of ways. At the late bud stage the stigma starts to elongate and can (rarely) pick up pollen on its way out of the anther cone. Later, after the blossom opens pollen shakes loose creating a mini dust cloud around the stigma. Anything that can jostle the blossom will create more of these little puffs of pollen. As far as making seeds goes the first pollen grains on the stigma usually gets to the ovules first when everything is working as it should, but mostif not all of the pollen matures after the blossom opens, high temps kill pollen, high humidity clumps it together so it won't fall out, calm days do not promote pollen release, etc.
The pollen matures over a period of time and is released only at certain times (mid morning to mid afternoon). Bees scent mark flowers with each visit so that they don't waste time with a flower that has just been stripped of pollen. That is why they seem to look at alot of flowers and not land or simply fly on by. This is also why they may skip over several plants and hit flowers quite far apart from each other, especially on days where the are a lot of bees around.
All showy flowers have evolved to be pollinated most efficiently in a certain way, usually through a specific action of an insect. Tomato flowers and some others are built to be "buzz pollinated", whereby a bee will vibrate the flower to shake pollen loose. These flowers usually have a funnel shaped anther cone (anthers fused or hooked together) with pollen released on the inside surface of the cone to shake past the stigma (tomatoes), or to be released through pores at the tip of the anthers (potatoes). Bees in tomato greenhouses are significantly (economically) more successful in setting fruit than any other method.
Honey bees, unlike most bee species, collect nectar to stockpile honey as a winter food source for the hive. While they are doing this they are also collecting protein rich pollen to feed their larvae, so they tend to ignore small flowers that don't have nectar. Most other bees just need the pollen for their young, and maybe a little nectar for their own food. They lay and egg in a chamber, stuff in as much pollen as is needed for complete development of the larvea once it hatches, and then seal off the chamber only to start the process over again. The more pollen they can collect the more eggs they can lay so they will work themselves to death, flying several hundred yards from nest to flowers round trip.
Will planting a few of any one variety close together cut down on cross pollination? I doubt it because the scent marking causes bees to skip over so many flowers. You would need distinct separate patches with a good number of plants for each bee to coat itself with pollen from only that patch. Number of plants (total number of flowers) would be more important than distance.
Cross pollination statistics:. In a mixed tomato garden cross pollination per fruit will range from 0% to (rarely) around 40%.
This often cited article gives a value of 2%-5%, but also mentions other studies with higher values. Because of the many factors involved (number of bees, weather conditions, etc) the value can shift wildy over the course of a season in any one garden (I get around 0%-5% for first fruits, and 20% mid season).
What is the easiest way to prevent cross polination? Bagging blossoms (actually bagging buds so that the blossoms open in the bag). Places like Walmart and crafts stores sell organza drawstring sachets for something (my masculinity might be threatened if I knew what people normally do with them). Just slip the bag over a truss of unopened buds and wait for small fruit to appear in the bag, remove the bag and tie a bright marker on the truss, and make if very clear to anyone in the area not to pick those fruit. I find it easier to cinch the bags down if I turn them inside out first, that way the cuff doesn't get in the way.
I suppose you could take a piece of tulle netting and some string and tie a make shift covering over a bud truss. I don't think that the bees will try very hard to squeeze into a mass of folded fabric.