Don't blend powders of differing makes. If you carefully and thoroughly blend similar powders, different lots, you'll just wind up with the same powder but with a uniform lot consistency now.
Also, know that commercial powders are of higher quality and are more stringent than military powders, so while they may vary slightly from lot to lot, they don't vary THAT much. You can expect one lot to perform similar to another, the changes should be minute. But if working up a new load and you want it to have as few variables as possible, a uniform lot isn't bad.
Personally I don't mix powders but I get it in bulk and generally have enough to load all my components and then some with that one lot.
For milspec ammo or plinking ammo or whatever you wanna call it, I disregard lots altogether.
