You want my crazy opinion? I actually think about this when choosing rugs: in the case of a fire, burning synthetic materials are way more damaging than something like cotton. So, in the event that your house catches on fire, the melting plastic in the pillows could cause some really damaging smoke.
I have to admit that I have polyester pillows in my house AND polyester curtains. They're "faux" silk. But I liked the swatches and I don't live in a historic house anymore... Don't judge me!
LOL but if those babies ever catch on fire...I'm toast.
If the combination looks like cotton and has a high thread count, go for it. I have some blend fitted sheets and I like it. Smooth, doesn't wrinkle as much as 100% cotton etc. Unfortunately some blends may look “cheap” so I’d pay attention to the fabric quality.