Believe it or not the risk of cross pollination isn't much higher with close spacing than with more traditional row spacings.

Tomatos generally do not cross pollinate readily as they self pollinate before the flowers even open. Peppers will cross pollinate readily even 50ft apart. Vine crops are generally insect pollinated so those also will readily cross pollinate even with a lot of space between them.

In other words, as a home gardener if you want to save seed and keep it from crossing you can't really rely on spacing to do it for you. You have to get into bagging blossoms or entire plants and with some things hand pollinate.

Of course if you don't mind some crossing and won't be trading seed with others then it makes no difference and you can end up with some very interesting plants.