The short answer is yes, you should be washing your workout clothes after every single workout. Even if you think you did not sweat too much or that your clothes do not stink, you probably still need to wash them after every use. The reason first is because even though you might not be sweating, your body is still likely producing odor causing bacteria through your skin. It is the body's natural reaction to an increased heart rate, any 'fight or flight' type experience in which adrenaline goes up, or when your body starts increasing its internal temperature in order to keep up with increased movement (i.e. you might feel yourself getting physically warm after you have started lifting weights, even if your body is not sweating or if you are not necessarily participating in some form of intense cardio vascular activity).
If you think your clothes are getting too worn too quickly after multiple washes, then you might want to consider investing in some heavier duty clothes that stand up to multiple wears and cycles through the washing machine. I know that is probably not want you want to hear, especially because nicer brands of running and exercise clothes can sometimes mean dropping some serious money. I would advise to definitely hit up the sales racks first, and keep an eye out for "out of season" clothes. even if you can't wear them now, you will be happy you snagged them for such a great price in a couple of months when you CAN wear them. Finally, check on what kind of laundry detergent you are using. There are some excellent sports laundry detergents on the market today that are designed to not only ensure that odors and bacteria caused by sweat are completely removed from clothing, but that they are gentle on your fabrics (since they know these will be clothes that get worn a lot and washed often.)