I've spoken to a UCC rep, who gave me a pretty detailed answer.
He said that older wax-based aluminum electrolytics could have safety issues with being mounted upside-down. If the wax warmed up, it could run to the bottom, cover the vent and resolidify. This would effectively prevent the vent from operating, potentially making a bigger boom in a failure condition.
He also said that more modern waxless caps still had potential issues with electrolyte leaking out through the vent. This is especially true if the cap isn't being run with high ripple currents. If the cap is warm, the high temperatures put pressure on the vent, creating a better seal.
Finally, he mentioned that scored PCB-mount caps could still leak electrolyte if mounted upside-down, and that they don't recommend it, but it's not nearly as serious a concern due to the smaller quantity of electrolyte in PCB caps. He didn't say so, but I'd expect that warmer caps would actually leak more electrolyte in this case, unlike the screw terminal caps.