I don't know if it's changed, but alot of people back in the DDR2 days would be screaming no and saying computers never work like that.
For me however, I've practically always ended up running stuff mixed in all kinds of ways.
Until my actual 2x8 set comes in, right now I'm running what was laying around because I just switched to DDR3.
I have 2x2 GB at 9-9-9-24 1.65 volts and a single 4GB stick (came with the mobo) at 9-9-9-24 1.5 volts. I'm running all of it, so the 4 gig stick isn't even dual channel.
Still runs fine. I've never had a problem with stuff being completely incompatible. In fact I only ever look bemusedly at the lists mobo makers give you as to what ram will work before ignoring it entirely.
Even if you can't get the advantage of it, because of the ram you're running more than likely at a higher volt, I'd get something low voltage.