Part of their scheme is coming off as a low-rank, non-powerful, non-threatening adventurers party, so hiding the powerful staff surely plays into that as well.
All that said, he has been hiding it since he arrived to the demon continent, before they even thought of forming an official guild party, so other reasons are at play as well you mention it, maybe it's also something the helps maintain it? Though I doubt that since Roxy herself is openly carrying her staff.