As someone who always requests het pairings, I don't even mention slash. I mean, slash doesn't bother me, but I'm requesting het, so I assume that my writer won't pair up my ship in a way I didn't request. I find the extra "no slash," redundant and grating to those who do write/read it. If they happen to go ODAO, then that's on them, not me. Mentioning no slash in a DNW, implies there is something squicky about it, and that is the last thing I would want anyone to think I believe.