It never hurts to drop the voltage to protect tubes. 50 ohms would drop 7.5 volts, and at the 0.15 amp draw of your tube string that would require the resistor to dissipate 1.125 watts: use a five watt wirewound to have it run cooler. You could even go 56 ohms. I also like to put a CL-90 inlet surge supressor in series with heater strings to slowly ramp them up. They'll start out around 60 volts and over twenty seconds or so the CL-90 will heat, its resistance drops, and the voltage comes up to just below line voltage. The CL-90 will drop another couple of volts so you'd be down to where you want to be. I get mine from Mouser, and need to order some more, BTW.
