I usually will advise people think of the convention using "warehouse" rules for footwear. Nothing open-toed or open-heeled. (though, I doubt anyone will need steel-toes)
Not only are sandals/flip flops not as orthapedically sound as a full comfortable shoe, but in high traffic areas it is very likely someone will step on you at some point. having someone step on the heel of your flip flop is horrible.
The above being said, I have really comfy sandals, and live in them during the summer. Unless I'm wearing my steel toed shoes, anyone stepping on my foot is going to hurt it, and the only issue I've had is a stroller nearly running over my foot.