A reason to put your HDV in an SD timeline is so that it can be monitored in SD while you work. It’s not a big deal.
Jeremy, not sure about what FCP 6 does, but I think most cameras enable downconverting from HDV to DV, I know my Canon A1 does. But, I still prefer to use native HDV, as it gives me the option of zooming in or panning and scanning if I need to. I’ve got just such a job in the house right now.