The lower keV x-rays are not useful for diagnostic purposes, because for
example, 10keV photons would be totally absorbed in your body, (still
very probably damaging your DNA), and basically none of it would make it
through to the cassette with the intensifying screen (fluorescent stuff)
and film, so it does not help with making an image of your innards. So,
to maximise the benefit and minimise the harm, medical x-ray machines
use relatively high voltages and are also required to include a filter
equivalent to a certain thickness of aluminium in the beam path between
the tube and the patient. This filter removes the lowest energy photons
(that are useless and harmful because they would be totally absorbed in
your body) without causing too much loss of the higher energy photons
that are still harmful but are also potentially useful because they have
some chance of passing through you and making an image.
All vacuum tubes will generate photons internally with some energy. Any
photons shorter in wavelength than roughly 300nm can damage your DNA,
and that corresponds to only a few volts on the anode. Whether the
vacuum tube will emit hazardous photons externally depends on whether
the photons can pass through the vacuum envelope of the tube. I would be
quite careful at 5kV or even below 5kV, if there is absolutely no
shielding other than a thin glass envelope. Whilst the tube envelope may
attenuate the x-rays by a large factor, on the other hand the anode
current, tube-to-human distance and exposure time with e.g. a RF
amplifier being serviced, might be much less favourable than in the case
of a medical x-ray. Medical x-rays usually use a few milliamps for a
second or less. Big amplifiers may run several hundreds of milliamperes
for hours. The other thing to be aware of is that many Geiger counters
are very (or totally) insensitive to 5keV photons. Whilst the Geiger
counter may tick furiously with the accelerating voltage set at 40kV,
and fall completely silent as the voltage is turned down to 5kV, that
does not necessarily mean that the x-rays have stopped, it might just
mean that they are now at a wavelength that the counter cannot detect.