If you put a little clorox in your dishwater when you wash them, you won't have to worry.
With a few holes poked in the bottom, the shallow stryofoam ones (2" or so deep) make great trays to start seeds in.
I often send "care packages" of food home with my kids and friends, and the disposable containers and trays are great for that.
I guess I'll be carrying home the Walmart trays from now on. The new Super Walmart has been open about a year and a half, and has run the two family owned grocery stores in our small town out of business. Now I have no butcher or green grocer to order special things for me...no more market made ground chuck or sausage...nobody to help me empty my cart or carry groceries to my car...I hate it.