It is more important to tie the screen and plate together through a proper dividing resistor network or screen dropping resistor such that the ratios are obtained under key down. For testing, use a 100 watt 100K ohm resistor with a slider tap and adjust until the screen voltage is about 150 under load. My guess is that that resistor should be in the range of 10K to 20K ohms for a great many tubes. The screen dropping helps to isolate the plate and screen circuits. Also the bypassess associated with the plate and screen dropping networks, plate chokes, and tank coils should be good so that proper rf bypassing to ground occurs.
Poor bypasses can ruin an oscillator or amplifier.
Nice, hefty buggers. HV transformers from microwave ovens may or may not be a good thing. They are cheap and lots of iron and copper. But, they are usually a single HV winding of about 2000vdc and a single LV filament winding of about 2.5 volts. You can use the HV winding on a full wave bridge but not both windings together for a tube style HV rectifier system because the insulation between the windings is marginal.