I got a question for those who ordered....

when i try to order a set of tires...it alwayz ask me to order valve stems..are they necessary??..dont all tire have them??..or is it just a way for them to cheat customers for their money?..they seem to have great prices on their tires...but the extra 12 bucks on valve stems realy kills the deal...question is....do i really need to order the valve stems with the tires....thanks again!