The question: "should you wash new Clothes especially new underwears you bought before wearing them?"

The answer: Although most of the clothes or items look brand new and fresh, most of them has had a germ-filled life before it reaches your closet. Remember, some clothes had probably gone on and off in fitting rooms in which clothing would become good place for skin cells that may contain antibiotic-resistant bacteria like MRSA. 

So, always toss them in the washer to play safe, particularly if you bought new underwears and swimsuits.