[Arran]

I don't think it is as critical as some would think. I've washed chassis, typically on hot days during the summer time, tuning condensers, padders, trimmers, everything, and did not remove any mica sheets, but I do loosen the screws first. Granted I am not immersing them or the chassis, nor am I subjecting them to any risk of mechanical damage, but I also rinse the chassis thoroughly, and let the whole thing dry for at least a week before powering up the set.
With regard to mica sheets, it is a good electrical insulator, but also will conduct heat since it's a mineral. They used to use it for windows in coal fired pot belly stoves at one time. In some areas of the world they used ceramic washers rather then mica, or as Phillips did, they made these funny cylindrical trimmers put of aluminum, where one would section screw down into the other. They also had a sort of trimmer that worked like a gimmick, except it was on a ceramic tube, and you would adjust the capacity by wrapping more or less wire around the outside.
Regards
Arran