Always man...if a HS coach wants to play zone based on a matchup issues or strategy at that level, that's fine (same in college)...but for youth and summer club...you always teach and play man...it's about getting kids better and as a RUFamily points out, it's about teaching and education...and the kids...
I'm not saying this is your intention or situation at all, so please don't take it that way, but at the youth level, winning doesn't matter (IMO)...it's about getting kids ready for HS, and then if they're capable...college at some level...summer clubs should always play man...and as a by product, if your teaching leads to wins...well then that's great too...
My two cents...which isn't worth much...