pre-washing (precuts) is seldom advised-
if you are set on doing it- a mesh bag would be advisable- but be prepared for your charms to no longer be 5" squares- and some may be quite (wonky) with pre-cuts it is recommended to use them as is- then wash the finished item-
if they are batiks/handdyes and bleeding may be an issue you can do a color test to see if any are going to run- and either pre-wash them or remove them from the mix-
but you will be happier in the long run if you do not pre-wash your pre-cuts=
many quilters do not purchase pre-cuts because it is best to not pre=wash them=if they are an (avid pre=washer)