As a rule of thumb I wash fabric for any bag I make if the finished bag will be washed. For example a pe bag or a gym bag.
I also tend to repurpose fabric quite a bit and upcycle fabric for my bags so I make bags from a loved ones piece of clothing or from a sentimental uniform for example in that case I will make sure that sure the clothing is clean and if there are any stains I can't remove I will fussy cut around them.
This is a project I did recently from a much loved coat.
There are obviously some fabrics you can't machine wash such leather or faux leather but if you feel they need to be cleaned before you start your project they can be sponged clean with warm soapy warm and left to dry before you start to cut out.
I hope this post has been helpful
Until next time.