We used to give my Great Grandmother a fair about of grief about what she did with her Ziploc bags. She would often times wash and re-use them. You'd see them hanging to dry just over the kitchen sink when you'd go over to visit... and while you were on the way to the freezer to get a chocolate chip cookie or two. We'd tell Gramo that "it's not World War II anymore... they aren't being rationed or anything..." She, despite our playful ribbing, carried on... frugally reusing those baggies. Here's the thing... she was right about doing it... even Ziploc folks say you should reuse your Ziploc bags.
They say that each bag should be reused "several" times, because, "Ziploc brand bags are made with high-quality, recyclable materials that are strong enough to be used again." They recommend hand-washing them with soap and water and then air-drying them... which is EXACTLY what Gramo did.
But there ARE instances when you should just throw them away like when they've held raw meat, fish, eggs, allergy triggering foods, or when something has spoiled inside. Basically, anytime it's a food safety issue. Again, that was Gramo's move.
It also may be difficult to clean bags that have held oils and "clingy sauces." But you should make an effort to re-use any bags that held dry foods or other easy-to-clean contents.
If you're curious, here's more information from People Magazine.
