Just plain old low bleed like for 50/50 would be my first choice.
Rayon isn't, by nature, stretchy. It's not like Lycra or Spandex. I do notice a little more of a tendency to pucker when too hot, so you may now have two reasons to mind your temps. No biggie.
Of course, if these are light colored shirts, then regular old plastisol, still minding the temps.