M.F.
Unless there's more than 10 years between #5 and #6 or you've had all of one sex and #6 is the opposite sex, I think a shower is inappropriate and comes across as "buy me stuff". Why not just have a celebration where you invite all the same friends and family, making them to come celebrate #6 with you, instead of a shower. No registry and specify no gifts on the invite. Lots of people will still want to bring a little something, but no one feels obligated that way. Congrats!