I'm bit cautious, so I prefer to keep them apart. Be it squash, zucchini, pepper plants, tomatoes, beans. some are self-polinators and its safe while others are not. Its bit trouble some to remember which is which so I do the lazy person thing and keep similar plants away from each other.

In some instances I'm not too concerned about cross polination so I plant next to each other like Serrano and Jalapenos. But my bell pepper sits far away from it. Beets, and other root vegetables, I mix them up unless I want to save the seeds for next year, those I'm pretty careful not to get them crossed.

Best thing to do is go to the library and check. I don't trust lot of people working at nurseries especially those from the big box store.