I hear a lot of things like this all the time. Fert salesmen and people who don't know any better refer to K as the "stress nutrient." But, no research has ever supported that notion. Most research involving K and stress report that when K is not deficient in the soil, plants tolerate stress better than when K is deficient. They go on to say that adding K when soil test K is not deficient does not improve stress tolerance.
So, if you want to be sure, test your soil and see where the K levels are. If they are deficient, you might benefit from K applications. If they aren't deficient, adding more K won't help you.