LOL, No showing will not slow down your motablism. Yes it relaxes muscles and that is a very very good thing, as well as getting rid of all the sweat that can cause some embarasing problems. You do want to try and wait until you stop pouring bullets before you hop into the shower, as that can just make you sweat more and need another shower.
I don't know who or where that myth came from that's one I haven't heard and I heard a lot of them from the *busy* women who thought they didn't have time to shower and used prefume instead. It sounds like something they would come up with to validate that they don't have time to wait a whole hour to shower then go to work.
Showering actually should havea better affect on your motablism as it relaxes the muscle so it makes it eaiser for the blood to get to place it needs and deliever the oxygen, so the motablism can do its thing and have a steady supply.
I can kind of see why this myth came about, as people sometimes think that motablism is directly related to how fast your heart beats and with a shower it would be able to slow down to normal, but your motablism isn't just in speed of the heart beat, it is a chemical reaction, that when you raise your hreart rate it speeds up and goes off a lot fast, because there is more stuff there quicker that it needs to make a reaction, but once you start that chemical reaction and your blood continues to flow in your body it takes quite a while to make it calm down and go back to normal. The only way you could slow this down would be to take drugs like narcotics, or sleeping pills or alcohol, or smoking. Stuff like that, where you actually put a chemical into your body to slow your whole system down and it even causes your nerve receptors to slow down or not to send things. So basically stoping the body from sending the chemicals needed in the chemical reaction.
So go take your shower after you workout, and don't be afraid it will harm your motablism, just stay way from any type of depressant.
Sorry for babbling I had to work threw this to see how the heck they got such a weird myth.
Kit