After a year of a tank, especially if it is your first tank and/or you're new to the hobby, or you bought the tank from someone else, your tank is likely to have collected a TON of over-fed dropped food, poop, etc in the sand, and all that stuff that sits in it and works its way down will eventually cause problems with nitrates and phosphates seeping into the tank. I know most people will say that it takes a lot longer than a year, but if you're moving your tank now, it's just easier to do it when you're moving the tank than do it in a year or a few when you really need to do it. I firmly believe that even with proper vacuming and a good cleanup/sand stiring crew, your sandbed will still hold nutrients in it, and eventually leak them back out. Almost anyone with an older tank has gone through it, especially if they overfed or didn't take proper care of the sand bed from the beginning.
All i'm saying, is that it's just good practice to clean it out, and if you've got the tank out and are moving it, why not just hose the sand down and let any accumulated crud run out of it, then just reseed it with a bag or two of your old sand. I just did this last week with a friend's tank when she moved it from someone else's place to hers, figured we might as well do it if the tank is empty then, since it won't be for a long while.
