Do I need a supplementary reviewer if the rater is GS 14 and senior rater is a LTC?

No. You have a Soldier in your rating scheme. Even if the rather and senior rater was reversed, you would be OK since as a Sergeant, your Senior Rater only needs to be a GS 09 or higher.

I have a supplementary reviewer and my rater is a CPT and senior rater is GS-14. The directorate SGM was the supplementary reviewer on my last NCOER. I'm not sure why - the only time I've had a supplementary reviewer was when I was at STRATCOM because I never had anyone Army in my rating chain. I had at points civilian, AF or Navy officer raters and then AF senior rater. That's what I assumed was the only time you needed a supplementary reviewer is if there's no Army in the rating chain. Not sure why it is where I am now.