Would there be a problem installing this in series with the power transformer primary, given that it will effect the bias supply as well as B+ supply on the other side? My guess is that since both will be eased to full voltage simultaneously, all would be good.
I always use them on the primary. It seems to help a lot of things; transformer, power switch, you can use a fast blow fuse. I like to use a lower cold resistance NTC, like a CL80, 140, 150 or 110. CL90 really slows things down, and makes the most heat.
I always use them in series with the Transformer primaries on my power amplifiers, but switch them out after the large inrush current has settled. Basically a Triac in series with 4 CL40 in series parallel being switched out by 4 parallel relay switches, have used hundreds of this combo over the years with no failures, bullet proof
I suspect that best practice depends on the circuit details. For example, you might prefer heaters to come up first and then B rises slowly to limit over voltage of B + when using choke loaded rectifiers ?