I looked in the Kaiser casting book and also in some older Alcoa books I have. I could not find a technical reason for the r.t. age prior to the artifical age. This procedure is the stated heat treatment for both C355-T6 and A356-T6 but not for standard 355 or 356. I did find a couple of obscure clues. One was that thin cast sections may end up being in a more highly supersaturated solution condition after solution treatment and will respond more quickly to natural ageing. This would suggest that the 8-12 hour or 24 hour room temperature age would cause higher mechanical properties to develop in the thinner sections of the castings during the natural ageing period and possibly, a casting would show less distortion in the thin to thick section transitions after artifical ageing. The Alcoa book made a statement that if higher hardness was desired, it was beneficial to skip the natural age period and go directly to the artifical ageing cycle. However, if you look at stated typical mech properties in the Kaiser book, the C355-T6 and A356-T6 alloys exhibit higher hardness than the standard versions of these alloys, which omit the natural age period.That, of course, is contrary to what the Alco book states. I also have several papers on the development of A357 alloy (which also has a r.t.age prior to artificial age) and I will look at these tomorrow to see if there is any mention of the benefits of the r.t. age.
