Whether you choose to do it when you are packing or unpacking, moving is a great time to declutter!! It's probably best to do it when you are packing, that way you save hauling stuff you no longer want. However, preparing to move can be a very busy time. Also, if you can't give it away before you leave, you are stuck with hauling it to your new home.
I, however, do usually do this when I am unpacking. Sometimes you get to a new home and realize some things you wanted to say just don't fit in anymore.
I keep out a box and toss stuff in as I unpack, that I find I don't want, no longer need or that just will not fit in at the new location. If I end up with a lot to get rid of, I try to group it. Household items, clothing, toys... put them into their own boxes. Then I post each box on freecycle as a take all grouping.
It's nice to declutter. It's nice to find people who can use the items. And freecycling in your new home is a great way to meet people in your new area!!

I look forward to everyone else's cleaning, decluttering, packing, unpacking, feng shui AND organizing tips!!