Apple makes the the computer and say it's best to install memory in pair...the computer comes with a pair of memory. Just follow the instructions from the maker.
However, you could certainly do as you will. To push my MacBook Pro (2008) to the edge by installing six (2 + 4) because, although it recognizes eight (4 + 4), the laptop actually becomes slower with eight...addressing issue (Apple Official max memory is 4). With the Intel processors, one can install the memory that's not a pair...works, no problem. Mine works like a champion with Mavericks and 6GB memory. For me, I just wanted to extend the life of my MacBook Pro.
Still, like stereo, memory is best installed in a pair.