Is it cast and ground which is often called "jig plate", also known by the Alcoa brand name MIC6?
If so it is not the best choice for producing load bearing machined aluminum parts, this is not it's intended purpose.
Being cast it machines very easily, we normally use a spray bottle with water and a small amount of dish soap as lube otherwise it will glue itself to an endmill.
It's advantages are stability and as bought has already been ground on both sides so it is rather flat and will remain so.
Last week I tossed in the scrap a 12' length of 3/8 thick" x 6" wide 6061 T6 aluminum bar stock, it was not even remotely flat and the parts made from it would never measure in. Fortunately there were 5 more of these in the stock rack that were flat enough for the job at hand.
The point being that a product that is intended for a particular use (in this case aluminum tooling plate) and has been manufactured as such is far superior to a product that has not been post processed, grinding and other methods included.