Answer: I'm going to answer the second half of the question first because I think it is very important. The dandelion in anyone's yard is not safe unless it has been positively identified, chemical sprays such as pesticides have not been used on the lawn, it is away from roadway pollution, it is not in an area where animals defecate and if it is not where there is the possibility of contamination from lead paint or other toxic chemicals. That said, the dandelion in your yard has just as much medicinal potential as a dandelion carefully nurtured in a field on a herb farm far away. Some might say it has more potential and that leads us into the other part of the question.
While local herbs may not be the best choice in every circumstance, I believe they are the best choice in many, if not most, circumstances. There are many reasons I believe this to be so.
*Using local herbs decreases environmental impact. Fossil fuels are used to transport herbs from other locations and, depending on our sources, may not always be grown in the most environmentally friendly way. When we use the weeds from our yard we are using something that would have been mowed anyway or, even worse, sprayed with herbicides which can be damaging to beneficial insects such as bees.
*Local herbs may be more suited to local diseases. Some herbalists believe the plants which grow in a particular location are suited to the common complaints of that region. One interesting example of this is how poison ivy and jewelweed grow in the same type of environment and jewelweed is considered one of the most effective herbs to use against poison ivy reaction.
*Gathering your own "medicine" can be part of the healing process. There have been multiple studies showing how a positive attitude can be beneficial to the healing process. Taking an active role in caring for your health can be very empowering. As you develop a relationship with the plants around you, your emotions are nourished just as the plants you use provide nourishment to your body.
*Familiarity with local herbs can be valuable in emergency situations. If you are caught in a situation where for some reason you don't have immediate access to medical care, it can be helpful to be familiar with local medicinal herbs. The application might be as simple as being at the park without a medicine kit and using a plantain leaf to soothe the scrape on your child's knee. Or it might be something more serious such as using yarrow to pack a profusely bleeding wound on the way to the emergency room.
*If you use herbs from your own yard rather than buying expensive, exotic herbs you can save a lot of money.
*There has been a lot of controversy lately about the regulation of vitamin supplements, herbs and other alternative products. If you use the herbs in your own yard you have the power to make your own choices about which herbs you want to use without worrying about your herb of choice being taken away from you by regulation.
*If you buy herbs from local farmers you support the local economy.
If you don't already use locally grown herbs I hope this post encourages you to take another look at the valuable plants growing at your very feet.