ICU,
The other posters have it right. Are you plinking with an AR or trying to hit small targets with a bolt gun?

I shoot mixed head stamp with my AR and it seems to do fine, but I wouldn't do it on a bolt gun. Here are some or the effects you can expect from mixed brass:
* different internal case volume - the effect will be differential pressure from the same powder charge, and this could be significant
* different flash hole dimensions - probably the same diameter, but some are drilled and some punched - you can mitigate this by unifroming and chamfering the flash holes
* different case wall thickness - in addition to the effects on case volume, this can make a significant difference on neck tension, especially if you are using bushing type dies, which "fix" the external dimension and drive all the variability in thickness to the inside. Some will have a better "grip" on the bullet than others, and the results won't be real pleasing.
* different "hardness" of the brass - take Winchester and Lapua, for instance. The former always seems quite hard to me and the latter is usually pretty soft. They'll spring back differently after sizing and you'll get some of the same effect as the neck tension comment above. Plus, the shoulder bump may be different so you may get some of the case volume effect.

I just don't know how you'd set up your dies for precision work with mixed cases, unless you oversize everything and then use deis with an expander ball, but it still is going to lead to high case to case variability. The whole reason most folks reload is to eliminate variability....
Elkbane