There's a huge difference between using a rubber underlayment for a children's play area, to minimize the danger of injury, and using rubber where edible plants may be grown.
Even if ornamentals (vs. edibles) are being grown, the ultimate health of the soil needs to be considered. The health of the pollinators, now under threat pretty much everywhere in the world, also needs to be considered (search "Colony Collapse Disorder" for the threat specific to honeybees, responsible for 40% of foods eaten in the United States). Toxins in tires will make their way into the ornamental plants, poisoning not only honeybees but other insects and birds which feed on the plants.
Choose wisely those materials which come into contact with the earth, the soil.
Sunset Zone 17, USDA Zone 9