humidity is best around 50-60%. rubber don't like it too dry, if not the liquid inside the rubber will escape to the surface and the rubber will become harden'ed & dried out, which result in cracking & brittleness. which includes some soft plastic.
too much humidity, humid might go into the rubber, and when the air dries up, will escape, this might mean weakened material as humid expands the rubber and shrinks when escape.
just get a hygrometer and put it in the cabinet. there's many choices, some are as cheap as $1.50, if you want the digital version with temperature/humidity/time/date & even rain prediction, you can get 1 for $10.
generally you don't want temperature to be too high or low, applies to all materials including diecasts. Too high, things start to melt, disintegrate, expand, too cold things starts to shrink & crack, and paint starts to crack & come off on diecasts as they expand/shrink.
also, room humidity & temperature doesn't always translate to the humidity & temp inside the cabinet or boxes as heat/humid tends to get trapped inside especially if you dust proof your cabinets.
