Top or bottom they have to be removed for brushing the chimney. At least if it was mounted up top in the liner at the flashing it would be a no brainer to take it out for cleaning the chimney. Easier than getting a lot of chimney caps off. If would be more prone to creosote buildup up top but then the chimney cap is a creosote factory too and I doubt that a chimney fire has ever originated thirty or forty feet from the fire box. Also, if one has to be installed I would darn sure rather it be supported by holes through the flashing than the walls of a .005 liner.