Didnt seem to matter to me as far as time. I do however push a pipe cleaner down the stem if it seems open after i cut it. Smaller diameter stems seal themselves well, but the big ones are like straws! If i top a biggun and dont close it up, seems the stem dries all the way to the next node down. Dont know if it matters, but screws and pipe cleaner doesnt hurt either. My advice. Top em whenver you want to.