If using a indirectly heated rectifier (5AR4), you can either tie the heater center tap to the cathode which is connected to the (+) side of the filter. OR, you can omit the center tap connection all together. I've seen both in some highly respected designs.

If using a rectifier like GZ34 where the cathode is tied to the heater internally, or a indirectly heated retifier, then the (+) side of the power supply filter is fed from the heater center tap.

Joe