Personally I think the growbag people are wrong - unless they're trying to get us to hold things back. I want my plants to have some kind of an established root system before they're trying to flower so would plant them as you have.
By the time they're flowering, the roots have filled any pots so I think this can only hold them back.
Anyway, you'll have a flower or 2 within a coupla weeks with most varieties.