Originally Posted by BobK

It's generally preferable to use the Y cord. If you just input to the left channel (or right) then you'll only get sound out of one speaker when you play it back on a stereo system (TV, computer).
If you use a single RCA cord without a Y adapter capture in mono and it's not an issue. When you convert the audio in your editor to AC3 or mpeg stereo your editor will just make an identical copy of the mono stream for the second channel.