First off guys, my terminology could be wrong but when I say extruded I am talking about T6 6061 sheets and bar etc that you can get online, and how it differs from mic6? Cast aluminum?

I read somewhere that that "regular" 6061 will warp if machined too thin due to it being prestressed? Wereas cast will not and be more true to size?

If someone can plaese shed some light and correct me that would be great. I am looking to mill some flat fixture plates in the near future and i was told to look at cast alum.