Polyfill and Airspace

Okay, so I know that polyfill is to make the enclosure "larger" than it is. My question is this: If I have a driver that requires, or performs best in .75 CuFt and my enclosure is .75, should I use polyfill? Will this change the Winisd tuning graphs and how if so?

I have made two fiberglass enclosures for my car and I just finished filling areas with expandable foam to bring it back down to .75. I'm ready to stuff with poly or just leave as is and mount the driver.

Please enlighten me....