Folks have been cutting spokes prior to rebuilding for eons, and so far there's been no documented hub stress problems. Some folks say de-tension first, but I'm not convinced it matters to the hub, though it might to the rim. As long as you're scrapping the rim anyway cut away. If you're concerned, you might cut all the spokes off one flange first, which effectively de-tensions the other - on the rear cut the left (less tight) flange first.

BTW- Stainless steel has good scrap value and recycles well, but not with the nipples attached, so unless you plan on later unthreading all the nipples, make your cut close the the nipples to yield the greatest amount of recyclable SS. As to where to recycle, many shops separate and recycle SS and aluminum, otherwise see who does it in your area. Where I am, it's the Boy Scouts.