Pre-soaking AS for 3 weeks or so will have the effect of developing bacterial colonies and thereby nitrifying the sediment. That is about 1000X more important than any KH/GH issue. It's not clear why you are even worried about KH/GH effects of the sediment actually...
I had to add another litre of AS to my tank, after the ammonia madness of my startup I soaked it in a bucket for a week, changing water every day. I then measured for ammonia after 24 hours and nothing was detected. So I agree with Clive, soak it. In fact, if I had my time again I would fish and plantless cycle the tank in the dark for 3 weeks until all Ammonia was gone. A little patience to save a LOT of aggro.
ADA say don't wash the AS, but I've not heard anything about not soaking it?