Synthetics do have some change in different weather. They are still plastic, so extremely hot weather makes them softer, (or even hot barrels can make the foreend softer) and cold weather can make the plastic more brittle .
Laminates are sort of like pressure treated wood only better. The glue used to fuse the alternating grain layers is heated and soaks into the wood under great pressure. This gives it fantastic strength and makes is almost impervious to weather. Even if the laquer is scratched terribly, the wood is layed up in alternating grain patterns so that it wont warp. And the glue/adhesive more or less is soaked into the wood itself, so wetness is kind of a non-factor. I'd only wipe them down to prevent metal corrosion. The surface can be re-finished like normal wood when it becomes too unbearable to look at. ( rifles with battle scars have their own ### appeal though if you ask me!!!)