Unless everything one builds is sized to fit within a display case, the models on the shelf will periodically have to be dusted (as well as cleaning the shelves) and that dusting can result in a few "incidents" where a part (or two) may come loose. I had let a few of these lapse, just setting the dislodged part next to the model until I could get around to fixing the damage. One flap each on my Devastators, a dive flap on my Dauntless, a drop tank and top canopy on my Lightning, a bomb that somehow fell out of my Liberator, a sidewinder that came off the rail of my F-16, the tailwheel that broke on my Corsair, the antenna wires I snapped off my Albatross, tthe landing gear I broke on my Avenger, the various boxes that I knocked free from a Sherman... you get the picture.
I finally decided to get them all repaired and back in proper order and realized just how much I really do like a kit that will fit into an easily dusted display box. It helps protect the kit when moving it around and keeps it completely dust free, not to mention being a clear barrier to anyone who may be tempted to pick it up without the understanding of how fragile they are.
I can't say that I have a real set schedule for the dusting to the non case, openly displayed kits, just whenever it begins to look too unkempt. I try to be careful handling them, only doing one shelf at the time and using a soft make-up brush to do the kits themselves, but there's always that chance. I know this has got to be a common dilemma and am curious as to your own frequency of clean off and do you tend to your "mishaps" immediately, or do you postpone?