Since there is no material for the mold to feed on like paper or wood, mold would not be a big issue. Still, it is a good idea to pull your frig out, vacuum under and around to prevent heat build up, and clean the drain pan and drain hose. I would think once or twice a year would be fine.
Our drain plugged up and I unplugged by blowing air back through it. Had to remove a panel in the freezer to get access so I could pour hot water down the drain from the top once I loosened things up. Also had to defrost everything and take everything out to sit in coolers. So it ended up being a big job but it was worth it since we no longer had overflow water on the floor! We were ready to get a new frig but did not have to. This only happened once in the 19 years we have had the frig.