I have seen clock designs like this one:
where the nixie tube leads are not fully inclosed. There is likely 145-170V on these lines. I am just wondering if it's safe to allow the leads to be exposed and what sort of consequences there might be if someone accidentally touches them.
That sort of exposed circuitry is not a real good idea. That said, as long as the HV is supplied by an isolated source such as a transformer, it's probably not a usually-lethal threat. The reason is that the HV is carried on conductors which are close together, and the most likely accident will involve a current path through the fingers of one hand. At 170 volts, this will be painful, and possibly result in burns, but there will be no path through something critical like the heart.
Of course, there's always the possibility of grabbing the board with both hands and getting unlucky. The problem with making things foolproof is that fools can be so very clever.
You need to enclose the circuitry in a suitable case.
Possibly something like this...
Or walnut and polished brass, if you like the steampunk aesthetic.
Don't forget to ground any exposed metalwork and protect the circuit with a fuse in the PSU.
