Let us know what the manufacturer says. As long as the cellulose is fire rated like fiberglass then I would hate to leave 18" of open space but the homeowner is the boss. Each of the cans should have a sticker somewhere that would state what its rating is and if they can/can't be covered with insulation.
Last time I looked, the home cheapos carried both types and I can see some unknowing homeowners saving a few bucks on the non-IC rated ones. Insulation installers should check.