It all depends on whether NW will *really* continue to fly during the strike -- the first tip off may be cancellation of late afternoon -- evening flights on Aug 19 so that all of their planes will be at hubs and not the end of spokes -- if there out of operation for a few weeks -- as happened in 1998 -- they'll be where they can be easily worked on.
Thus flights to places like DTW, MEM or MSP are probably OK.
Bob H