I've done a few lasercut parts where the kerf width mattered. I asked the cutting house, and they said for the machine they would use for my parts, the kerf would be .014" wide. It was.
I think larger photon cannons and waterjets leave a slightly wider kerf, but the best answer is to talk to the actual vendor you'll use, and ask them for a number for your material and thickness.
Mike Halloran
Pembroke Pines, FL, USA