I never mist my orchids as a group "overhead" (except maybe very lightly which does enhance humidity in and around plants that are grouped together). Not heavily, though. Too many nooks and crannies that water might lodge in. I do mist exposed roots of cattleyas, phals and oncidiums at least daily. It seems to help roots keep growing - especially under warm HID lights. Misting individual thicker leaved sympodial plants like cattleyas also discourages certain pests from taking hold and keeps leaves clean. I never mist leaves of paphs or phals - too much risk of crown rot. With pleurothallids, which would be grown in high humidity environment anyway, misting is also beneficial since moisture will last longer in a humid atmosphere, even with circulating air.