Should We Still Install Matched Pairs of RAM? When I say "still", I'm not actually talking about current generation Macs. I'm specifically talking about 2012's. Is there still benefit to installing matched pairs or are those days long gone? 

I have two 2012 Macs with 4 GB each. I just got my hands on 8 GB from a 2012 iMac. So the question is, which Mac do I upgrade to 8 GB?

...but I could also do 2 GB + 4 GB in each Mac, bringing each Mac to 6 GB. 
Way back when, it was always said to install in matched pairs, otherwise the RAM run's "slow". But is this still the case by 2012? Even if it is true, just how much slower is it? It still may be worthwhile to run unmatched pairs to get an extra 2 GB in each machine.
No - even in the core 2 days, it was almost always faster to have more RAM that wasn’t matched compared to less RAM that was matched. I believe the penalty was 3% range - so you could test it in a benchmark, but real world - more RAM wins.

The only way matched ram would matter was in a perfect tie. Say two chips of 4 compared to four chips of 2. Always go with more RAM when you have capacity unless you are on a power budget - mobile devices on battery have to power all RAM and if you provision too much, you waste battery. 

These days, the memory controllers are so good, I don’t think you can even benchmark differences due to a mix of memory sizes or locations.