for best performance, yes they should be matched.  matching makes sure the tubes have similar cathode currents.  matching will definitely get you more power, and some people say a better sound.  better is all relative in my book, but power is measurable.

i would say though mixed tubes won't hurt the amp, although it could hurt the tube.  since the dc30 is cathode biased all the tubes are biased by the same resistors, so if they are different one can be biased too hot while the other too cold.  it's not that much to get biased tubes, so its probably worth it.  i like the jj tubes.  good price for good performance.  its not worth it for me to blow a bunch of money on power tubes with the way the matchless burns through them.  tubedepot will make sure you get a matched quad.  just ask for it in the comments section when you checkout.