Your decision to mix different powder lots is your own, although you should know that there are pitfalls to consider if you do mix different containers of the same powder together. . .
When mixing, agitating, sifting, etc., any powder with surface-applied burn-controlling retardant, be very careful not to overdo any of your mixing activities as the burn-controlling retardant can be reduced/removed during the process giving you an entirely new burn rate which you may find is extremely dangerous to your health if you are unaware of this fact. It's a lot easier than you think to affect a burn rate in this manner and it also happens in a lot less time than you might think.
As previously mentioned, periodically the actual manufacturer of a given powder is changed and even if the end product looks the same, important characteristics of the powder may not actually be identical. You have a leg up on the situation if the actual physical powder is a different color or shape so you would know not to throw that dis-similar powder into your mix no matter what else you do.
Always make sure that you are only mixing one type of powder if you choose to combine partial containers, don't let a stray powder type get in with several partial containers of another powder.
Do many handloaders mix different powder lots? Sure they do, but that doesn't exactly make it a great idea. You are gambling with the percentages that you won't be the one to have a serious, problematic and/or injurious result. Is a small convenience worth the chance of injuring yourself or. . .God forbid. . .someone else which may also include your own family members. The great thing about being over 21 is that you get to decide these things for yourself. The bad thing about being over 21 is that you are responsible to yourself and others to make the correct and proper decision - if you don't, there are doctors, hospitals, morticians, prosthetic specialists and lawyers to point out your errors of judgment. This is one situation where lawyers may actually be the best choice, but I digress. . .
Are these extreme examples, scare tactics and preaching by an overly cautious guy like me? Maybe, but I doubt it. Being a selfish guy, I'd like to have the chance to meet all you potential powder-mixers at the range some day and during that time, I'd rather not have to hear about how you lost an eye, a child, or your family fortune because you made a mistake in judgment. I personally like great stories about hunting, shooting and outdoors camaraderie. . .the kind of stories you'll be telling if you DON'T have an accident.
You're probably wondering why I have such a great interest in your health. It is because I have a great friend who made the wrong choice when it came to combining some different powder lots a while back - you can't always be sure that when you take a risk that you won't be the one out of "X" number of folks who comes up a little short in the luck category. He still has the scars and structural damage to the face he has to wear out in public every day and he brings the same face home to his family every day as well. I believe if you ask him what his opinion is these days, he would now most likely rather stick with the safest thing which is not to combine the powder lots - but his is just one of many opinions, I'm sure you have your own. When it comes to handloading and shooting safety, I hope you will choose your path wisely.
Javelina