Although I haven't bought specifically "for stash" yet, I have bought additional prints in a line "to use with this project if I need it." I like having options.
Having said that, I recently added a couple more fabrics for a Blooming Nine Patch project- 1 yard each, and "while I was in there" (Etsy) I added "a-couple-of-yards-of-this-coordinating-print-just-in-case-I-decide-I-like-it-better-than-one-of-the-others-or-maybe-I-won't-use-it-at-all-on-this-project-but-I-like-it"
That's essentially my stash-building mantra at the moment.