What comes out can also get in. I had a muffler shop weld shut any holes in my header pipe. I think these holes where do to old age. Then I powder coated it to prevent rust and corrosion. No holes in my stock 87 Hurricane mufflers that I see. I also stuff a rag in the tops of my mufflers when giving the girl a bath to prevent water getting in.

 

These holes are not in the header pipes. If your mufflers came with the vent holes and you weld them shut, it will accelerate the corrosion inside. Nothing can or will get in those holes.