I completely disagree, as long as you're not mixing brands in the same channel but even than it's circumstantial on speed and timings.
Otherwise you should have no issues but remember if you get a slower type than what you have now all the ram will clock to the lowest speed.
So make sure to get a speed either as good as what you have or better or you may see a decrease in performance.
I am mixing ram right now in my desktop and I have no issues whatsoever. One channel is 2x2 corsair the other is 2x1 kingston.