For the most part, no. But in some cases, yes.
In my experience,(almost 2000 models built), flat cast kits that warped and were then flattened tend to stay flat if assembled straight away. With one major caveat, there are some cast sides out there that have cupped or dished. They can be very challenging to flatten and will return to the undesired shape. It seems to be an issue with the type of resin used.
There are a few one piece body castings out there that will never be able to be corrected. They are a certain vintage of passenger body� castings and flat car castings. There seems to be a stability issue with the thin sides and very thick roofs or floors. Again , I believe it's an issue of the material used for the casting.

Pierre Oliver
www.elgincarshops.com
www.yarmouthmodelworks.com