You need the BBC's unless you have either a SWM8 switch or a SWMSlimline dish. The SWM8 is normally used only in multifamily homes and the SWMSlimline dish is only in limited areas at this time so the chance are that you need the BBC's.
Maybe some of the posts have read are old. The directv site lists the B-Band converters needed for HR20 receivers. Solid Signal where I am looking at getting my equipment from also lists B-Band converters for HR20. Nothing mentioned for HR21. I can't even find a mention of the swmslimline dish on the directv site. I purchased 2 HR21 receivers, a slimline dish and the Zinwell WB68 mulitiswitch. I just need to figure out if I will need the B-Band Converters as well now so I can order it all at once.