I've been loading warm (+P) 38 loads in mixed brass for twenty years and have never been able to detect any difference. The only way I was able to detect anything weird was with the WCC +P+ brass, it was a tad heavier than the standard velocity stuff, but that could have been the nickle plating. I wouldn't worry about using mixed brass, but if you're going to load above +P levels, I'd use 357 brass.
Papajohn