If your garden is in the front yard of a house located close to a busy street in an old neighborhood, maybe so (not sure of the risk). But in the U.S., cars manufactured after 1975 used unleaded gas, and leaded gas was no longer sold after 1995. So I would think that in any subdivision built after 1975, the risk of lead in the soil caused by car emissions is very low. If any kind of industrial facility was previously on your land, you should test for more than lead.