First time; Registering for gifts takes time. Time you could spend in a gazillion different ways. More likely than not you'll find yourself wandering around stores with a zapper in hand, selecting items without much discernment. Really, do you need a silver punch bowl?
Second, gifts must be stored somewhere. Look around your current home -- where will you put the 100 plus gifts you are likely to receive on your wedding day. Will it feel cramped, putting pressure on you to move into larger living space? Figure out what you pay per square foot in your rental or home, then you'll get a better idea of what it will cost to store the china set you likely won't use but once a year if at all. Remember, space is not cheap, and it takes time to maintain. My husband and I managed to live in an 800 square foot apartment for the first year and a half of our marriage, moving only when I was 6 months pregnant. And our move was to a two bedroom duplex. It was only after the birth of our second child that we moved into a small-ish home. People thought we were nuts. But we didn't have much stuff, we could keep life simple. As a result, we lived debt free for just under 5 years of marriage. Marriage without financial stress is a wonderful thing, much more enjoyable than new towels from Target.
Third, receiving gifts takes money. Upon receiving a gift you should properly thank the giver; you'll need thank you cards and postage. They are not free. The cost adds up, faster than you might think.
In practice, this is how declining gifts might look. On our marriage announcement we stated something on the order of "your gift of well wishes and kind words is appreciated, no other gifts are necessary". Most people honored our wish. We did not view our wedding as a way to "cash in" and score some great stuff. Some people did send a gift, and others gave us cash. My parents gave us the amount they would have spent on the wedding, which came as a delightful surprise. We put it into savings. Another twist would be to ask people to honor your vows with a donation to a charity you have selected; what charities reflect your values and dreams? We didn't think of this in time for our wedding, but on each anniversary we donate money to the Arbor Day Foundation instead of purchasing gifts for one another. Get creative. Get inspired. Walk away from convention. You'll save money, do something nice for the earth, and feel good about yourselves. At least we did.