I can't tell you that I'm right as I don't remember what I've read on that but in my mind, if you wash out with a high-pressure sprayer on the shirt side, you may eat away at the edges of what should be your perfectly formed stencil image. If it's going to be beat up, then the squeegee side would make sense to spray out. Having said that, It's not what I did. LOL. I've always wet boyther sides, let it set for 30 seconds to a minute...and sprayed out from the shirt side. Not sure why since again, I don't remember what I've read on that to confirm.
Sorry. I don't think I helped much with that. LOL.