I use RG6 quite a bit for ham work, and the cable I buy uses a HDC centre
conductor. I would avoid CCS for lower HF.

For what it's worth, I looked up the specs on the Carol C5785 that is
locally available at Home Depot here in the States. It's quad-shield
RG-6 and they list the losses down to 1MHz

1MHz .26dB/100ft
10MHz .81dB/100ft
50MHz 1.46dB/100ft

According to your calculator for RG-6/U it should be
..19
..6
1.37

As a percentage difference in dB (boy that's a bad unit) it's actually
a good bit more loss at 1 and 10MHz, but in a practical sense it's
probably pretty negligible.

So unless you're going 1000 feet to transmitting antennas on mid to
low HF, I doubt it's a worry. I don't know what the price
differential is between CCS and hard drawn but i do know that cable
with about the same loss as RG-213 that costs 12 cents a foot is
pretty attractive.

As far as the original post, I picked my stuff up and made a bunch of
twelfth-wave transformers for it and that seemed to work out fine, but
I guess they're less sensitive if you put one at each end because they
would tend to match to whatever cable you're making the transformers
out of and they're very broadband too.

As far as velocity factor goes, I measured mine before I started
cutting (I built XFMRS for 2m and 70cm so I had to be sort of
accurate. It worked).

73,
Dan