My post is just presuming that I'm a very experienced model builder so all common sense things are already covered. Decals down before weathering? Of course. sealed with rest of the model? Of course.
The Sherman looks good. A light base coat as mentioned may help. The problem is the white will take weathering different than the paint even if its weathered the same and its all sealed the same, as the decals are usually brilliant white.
I always opt to paint if its a shape.............obviously if its a US star or a number of any kind, one can't really paint it on like you could a rectangle.