Mixing Cotton Fabrics.....HELP!! :)

I am new to this sewing stuff and am confused about something.....If I make a dress out of 65/35 poly cotton blend and then add a trim at the bottom in 100 percent cotton....what will the dress come out like after washing?  LOL...this might seem ridiculous but I am wondering if I should pre-shrink the 100 cotton before adding it to the blend....any suggestion would be greatly appreciated!

I would pre-shrink whenever possible especially in this case. It depends how big of a cotton trim you are adding. If it's just a 1/2" bias strip, it should be fine. If it's a thicker band, then you might have some wrinkles and different drape. You might want to keep it as thin as possible to avoid any issues.