You know, when I began reading cards, I never did this as I had never heard of doing this before. Then, I came here to A.T. and heard about it and I did try to do this to a few decks I already had. But, it became more of a chore and not always convenient to do, etc. so I soon got away from that too.
I realized that it really isn't necessary, unless you personally wish to do it. In that case, there are a number of things you can do.
Many people like to burn sage and smoke the cards. I did try this--but couldn't stand the aroma of burning sage--it smells a lot like pot. Others put crystals on the deck and leave it in a full moon, etc. etc. I've done this as well. I do still once in a great while, especially under a full moon. I don't think it really does anything much to it though. Its more of a superstition, I think now, but sometimes it just makes ME feel good to do this.
But on a regular basis--I'm beginning to think that just using the deck yourself and getting use to it and getting to know it, is the best way to dedicate it for its purpose!
I've received many various tarot decks over the years--some brand spanking new and some used. My #1 reading deck was a gift from a customer of mine and she had found it in the 2nd hand shop. We have no idea who owned it before--but it came with really good energy. Maybe due to the person who did own it before or perhaps because it was a gift to me from a very lovely lady.
Who knows? But, it feels comfortable in my hands and it was a well worn deck when I received it. So, I think using the decks over and over again is what helps to give the deck its very special feeling and dedicating it to the job was meant to do.