I personaly rather build a wheel with the manufactures own threads since rolled is stronger than cut ones. That being said my 4cross hardtail has a funny spoke size that DT alpines didnt come in. I cut them with a quality pair of cutters, then using a fine grit grinding wheel made the sure the end of the cut was square and free of any burs. I use a Hozan cuter at work when I have too. Make sure you have it set to propery cut the thread for the right guage (14 - 2mm 15 - 1.8mm) and use lots of lube. Setting the lenth of the cut can be a pain I use chainring spacers on the outter side of my cutter so it only threads about 9mm of each spoke before it bottoms out. Spokes break 3 ways, Head popping, nipple breaking or the spoke failing at the last thread from it not being into the threads of the nipple. If you have butted spokes your idea is useless since youll be cutting the spoke down into where it becomes narrower.
Measure twice and be prepaired to re thread a few of them. Good luck!