Pi stacking would most likely involve the rings: tryptophan, tyrosine, and phenylalanine. In Foldit, it's not clear whether there's any special scoring for pi stacking. There is no option to visualize pi stacking. Many player designs seem to contain potential pi stacking, for example having two tryptophans adjacent to each other. So it's possible that general scoring in Foldit recognizes that pi stacking is favorable. Thanks to veteran player Susume for pointing out pi stacking.