I bought an Olympus Digital Voice recorder, Model VN-6200PC. I carry it on a lanyard around my neck, often tucked into a breast pocket. At the maximum fidelity setting it will record for 71 hours (or until the batteries give out). I have worn it in a shirt pocket under a jacket, and I still get a clearly audible recording. Like others, I always carry it when I OC/CC, and I turn it on before I leave the house. If nothing happens, I erase it all, after I return home.
Is it necessary? That is a decision you must make. However, before making it, you should read the entire thread about Skidmark's "encounter":
http://forum.opencarry.org/forums/showthread.php?83848-**-Legal-Defense-fund-for-Skidmark-**
He may or may not have had a DVR, but we are all hoping that he did. If it happened to you, wouldn't you want to be able to play back the "ACTUAL" encounter for the judge?