I know in bottle neck cases that are fire formed are usually better.. does this apply to straight wall also?  I'd say no, but wondering if anyone has seen otherwise?  Specifically interested in 44 Mag, 45 ACP, and 444 Marlin.  Assume same brass headstamp & lot..

No, it does not apply.

Full length size straight wall and load away.

My 45 APC virgin starline brass is gtg right out of the bag.  It goes thru the size die, but it would be fine without.

I guess my real question is, is there any reason I shouldn't mix new brass with used brass.. would it make a difference in point of impact?

I usually do run new brass thru the sizer.

I try to keep all brass separated by the number of times fired. This is especially true for my bottle necked rifle brass. Straight walled pistol brass never needs trimming and gets tossed as soon as the neck cracks. Keeping stuff together (by number of times fired) gives me a heads-up when it's time to chuck the entire batch. Insipient case head separations on some rifle cases tells me the whole batch is suspect and should be recycled.

I have been told that low velocity pistol brass should be kept and loaded the same every time. That you shouldn't mix full power brass with target loaded brass. I shoot 90% or more of my .41 Magnum rounds using lead 215 grain bullets at a muzzle velocity of 960 fps +/-. These are never reloaded with full power 1300 fps jacketed loads. I keep them separate and reload them based on their first firing pressures. Whether this needs to be done I have zero idea. The notion that one firing at 22,000 psi which is then followed by a load using 35,000 psi shouldn't, in my mind, make a difference. It would seem to me that the brass should be able to handle anything up to and including SAAMI maximum. Anything less should be easier on the case.

Whether this is an old wives tale which has no basis in fact doesn't really matter. I have followed this procedure and since my overwhelming majority of shots are fired using light loads I really haven't suffered any consequences. It's been said that the brass has a memory and reacts according to the pressures it's been subjected to. I feel they give the brass more credit than it deserves. It's a dumb piece of metal that will fail over time, sometimes quickly sometimes slowly, depending on how violent a life it's led.  

Range pick-up brass is the most suspect. Reloaders who think it's toast because of too many firings, too hot a load etc. leave them behind the same as people who never reload. You never know what your getting unless you have to remove the military crimp.

9mm is typically thought of as a straight-wall case, but if you take a look at the case diagram, it's actually (very slightly) tapered.

I don't remember brands, but some factory-new brass comes tapered, and some does not.

Then when you run it through the typical carbide die, it ends up perfectly cylindrical with a much smaller diameter, you blow out the mouth, and shove in a bullet, expanding the case as you go.

So yeah, you could have some case volume inconsistencies with unfired vs reloaded brass.

Whether that'll make any difference...

I've never noticed any difference but I don't usually try to shoot 1 MOA groups with straight wall case calibers.

I do load for scoped 500 S&W handguns and 45-70 rifle (peep sights) and never noticed any difference with them as well.

Motor

The biggest killer of straightwall case accuracy seems to be the modern carbide sizing die. Most seem to size cases wayyy too tight, all the way to the base. The original steel sizing dies were slightly tapered to the case mouth. This left you with a larger base to help center the case in the chamber. Redding has picked up the two diameter case idea with their dual carbide sizing die. I suppose with a little measurement and calculating, it should be able to fine tune case sizing for long range shooting. Assuming of course, that Redding would make you a custom sizing die.

Brand new brass often has banged up case mouths.  That means you should full length resize those cases just as you'd size fired cases.

The "fire formed brass is 'better'" thing comes from long range shooters with bolt action rifles.  It's "better" because once it's formed to that rifle's chamber you only need to bump the shoulder back a fraction and size the neck.  Unless you're ready to geld gnats at 500 yards, this truism isn't particularly useful.