I would say probably not, unless you can do that repeatedly and in a consistent manner every time you do a water change to keep parameters stable. Achieving that would be really hard, I think (as I'll explain); so it mostly sounds like a recipe for dangerously unstable KH levels to me. You'd have to let the boiled water cool down every single time, and you just can't be sure of exactly how much the boiling will lower your KH, how to make that amount standard, etc. (at least not without a lab environment)... Ultimately, I really don't think it would be worth all the effort. If you want a lower KH, the best option is to use RO water and remineralize it yourself. Second best would probably be using something like peat that you can easily use a set amount of and at least somewhat control.