You shouldn't worry too much about toxic plants, because plants are natural thing, and dogs have a very highly developed sense of smell. Toxic plants usually have unpleasant smell to dogs and they won't eat then (at least not enough to get poisoned). Yes, there are exceptions to this rule, but in most cases dogs don't get poisoned so easy by toxic plants.
Man made toxins are however far more dangerous because they don't exist in the nature, and evolution didn't have time to develop mechanisms to detect them.
There are hoaxes all over the Internet about toxicity of some plants as well. Garlic is just one of examples. The hoax about garlic toxicity is based on a Japanese study done on 8 dogs. 4 dogs were treated with garlic extract, while 4 other dogs were treated with water. Dogs treated with garlic extract were given equivalent of 5 grams of garlic per 1 kilogram of body weight every day, directly into stomach, with a tube. To translate this to more acceptable measuring system, we could say that 20 lb dogs were ingesting 30 average garlic cloves every day.
After 10 days of the study, blood test of dogs treated with garlic showed light decrease in the number of red blood cells, but there were no visible symptoms of anemia.
Now, who in the world would feed their 20 lb dog with 30 garlic cloves every day? I doubt any dog would even want to eat that. That's why scientist had to use a tube to give garlic extract to dogs directly in the stomach, avoiding sensory organs in their mouth. Imagine yourself eating the same amount of garlic every day. For a 150 lb person that would be 225 garlic cloves every day.
This amount of garlic is, of course, toxic for humans. But when we say garlic, everyone will tell you it's healthy. That's because the amount of garlic we eat is small comparing to our body weight. The same thing applies to dogs. If given in small amount, garlic promotes healthy immune system.
There are many things that are toxic depending on the amount taken. For example, we all know that Vitamin A is necessary! There is no life without it. But if taken in higher dose it becomes very toxic. The same thing with Vitamin D, Selenium, and many more other nutrients.