I've done that as well. Used X as a placeholder.
Also, if I get a yellow tile I try to pick a word that has that letter in two different places. For instance, if I get a yellow E in the last spot on my first guess, I will go with a word like EVENT on my next guess to see if I can place that E correctly. Often times I end up getting additional green or yellow tiles. It's risky though. If E turns out to be the second letter and there are no V's, N's or T's, I didn't really help myself other than eliminating those letters and placement possibilities.