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For plinking, unless your particular gun demonstrates a preference, it doesn't make any real difference. For any loads heavier, or for ammo for a serious purpose, you are better off to seperate by headstamp, and lot(if possible). For max loads, uniform cases are a must. Target shooters go so far as to weigh each case, and reject any that fall too far out side an average, as this indicates a difference in the internal volume.

More critical to the loading process is ensuring that your cases are all the same length. Cases significantly shorter than the average will not get enough belling, or crimp, and cases longer will get too much. Too much can result in a damaged and unusable round. Differing levels of crimp will also affect the accuracy of the ammo.

Loading cases of differeing length can be done, but it is more tedious. If you don't have a trimmer, you should sort the cases into batches by length, using a caliper. If you don't have a caliper, get one before you get into serious reloading. They are not all that expensive, and are indispensible for a great many things. A case length gauge is handy and useful, but it does not replace a caliper.

After you have sorted your cases by length, you can load them, adjusting the the expander and the seat/crimp dies for each different batch compensating for the differences in length.

If you are using range pick up brass, inspect it carefully. In particular, look for things like off center flash holes. These can break a decapping pin in an extreme. "Off brand" brass is notorious for this, but it can happen even with cases from major manufacturers. But not very often.