Wherefore is "For what reason or why". Juliet is not asking where Romeo is physically, but wondering why does he need to be a Montague. So yes you can interpre

Wherefore is "For what reason or why". Juliet is not asking where Romeo is physically, but wondering why does he need to be a Montague. So yes you can interpret it as "for what reason or why are you going to be" (this thing she will now explain).