In general you can, I know quite a few people who do. You wouldn't want to use them though if you have sprayed pesticides in that area though, and probably not if you live in an area with a lot of pollution as they will absorb those toxins and you would be ingesting them with anything you did with such herbs. You can use the leaves either fresh or dried. If you are using them in a salad, you will want to use the smaller leaves as they are less bitter than the large ones. The leaves and roots do taste different as well when used in teas. The roasted and unroasted roots also taste differently. When roasted they have a strong coffee flavor and scent, but unroasted they do not. I use them unroasted because I don't like the taste of coffee. You will want make sure you are using the right plant too. There are plants that look similar to dandelions but are completely different plants, some are toxic.
If you have allergies you might want to try parsley (Petroselinum crispum) tea. Parsley is a natural antihistamine. It helps with all kinds of allergies. I know a lady that was taking for other benefits and realized that her hay fever, which had been horrible, was gone. Over time it also improved her allergies to tape, pine, and nickel. A good reference book is 20,000 Secrets of Tea by Victoria Zak. It's available as both paperback and ebook. When looking at herbs to use you also want to check all the things they are used for for indications of when not to use them. Sage (Salvia officinalis) is good for many things, but one of it's actions is to help control blood sugar; so if a person is taking insulin to control their diabetes they would not want to take sage medicinally unless under careful watch by their doctor. The amounts used in cooking are normally fine though.