I reuse mine. They are the first things I clean at the end of brew day. It's a good idea to save at least a couple for emergencies. I accidentally tore one one time and had to use the one from my previous batch. As Yooper commented, I just turn the bags inside out and soap and rinse them out really well.
I've also done a couple of BIAB batches using the 5 gallon paint strainer bags. If the one gallon versions are made of the same material as the 5, then I'll like them better than the muslin bags from Northern Brewer as they will be more robust and easier to clean.
"There is no right way to brew. The journey to finding out what works best for you is 99% of the hobby. Ethanol is the other 1%" - Bobby M