Probably adds a bit of support, and ensures alignment so the fin tabs fit in between the rings, but not super-neccessary. As long as your properly fillet the rings you should be fine.

The rear spacer ring is required to properly fit the motor retainer. Didn't realize that when I built an earlier PSII kit.

FWIW, I used the suggested paint job on my Scion and it came out looking pretty spiffy.

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NAR #27085 - Oregon Rocketry - SAM