There are no stead fast rules about poly or cotton. Poly thread ripping cotton fabric is an old myth. If you go to Superior Threads website he has very useful information about poly vs. cotton threads. Personally, I prefer poly over cotton for several reasons less lint, colorfast, easier to tension, doesn't change due to weather conditions, AND it does not shrink in the wash! I like using Maxi Lock thread for piecing because it is a finer thread and there is less of a bump when pressing seams. For machine quilting I use a lot of Maxi Lock, So Fine and Bottom Line threads.
You just need to find which works best for you. If you prefer the poly threads then use them. Whenever someone says you should be using cotton thread just smile and say, "Thank you."
The reason people think that poly thread will rip the cotton fabric is over time cotton fabric (being a natural fiber) degrades over time and with use and washing. Poly thread being a man made fiber does not.