Welcome to the board, Ro V.
I've been quilting for about 15 years and sewing for 50+. So, I have lots of thread..... 100% cotton, cotton/polyester blends, 100% polyester, and silk. I do love appliqueing with silk because it glides through the fabric so easily but I use cotton or cotton/polyester at times, too. I try to stay away from 100% poly because the thread is so much stronger than the fabric and I've heard that it can cut the fabric over time. I've seen that happen on really old quilts that got used and washed a lot.
I think the color of thread you use is more important than using only silk. I have so much thread that I can match the color of the applique piece perfectly and the thread just seems to disappear.... that's really important to me. I don't want those stitches to show and distract from the applique piece. The quilt in my avatar is all appliqued and most of the thread I used is 100% cotton..... because it matched the pieces. I may use 20 or 30 different colors of thread.
I'm sure you will get lots of others giving their opinion.... and this post is just mine. So, experiment with your applique, wash it a lot, and decide for yourself....... and happy quilting.
Nanna