Yes you do need the extension center tube, otherwise light will reach the reels when you pour in the chemicals as the tube forms part of the light seal. Are you using a Jobo rotary processor? If so, even if you get the chemicals in without fogging the film, without the rod the reels will float about and development is likely to be uneven. It might work for inversion processing, but it will take a lot of chemical to ensure all the film is covered. Hope this helps!
Allan