Since plastic has not been around for very long, and the risk of plasticizers and other chemicals found in plastic to human health remains uncertain, it's probably a good idea to err on the side of caution, just in case. Scientists recommend that people follow precautions to prevent exposure to chemicals found in certain plastics that may migrate into foods when heated in the microwave:

Choose microwave-safe plastic wrap and never let it directly contact food. If you prefer to avoid using plastic wrap, try waxed or parchment paper instead. Only use plastic containers that have been designated as microwave-safe. If possible, choose ones made of polyethylene, which is plasticizer-free. Otherwise, use microwaveable glass and ceramic cookware. Never use microwave convenience food trays and containers more than once. Do not microwave plastic containers used for cold food storage. They often melt and warp because they are not designed to withstand the high heat of microwaving. Avoid microwaving food in freezer cartons and on styrofoam trays.