Ironing the hem before sewing makes all the difference. Don't try to fold and sew in one motion...if it works at all, it will be uneven.
I use a Eurosteam steamer/iron that won't harm ANY fabrics, but makes a nice sharp crease in the hem.
A normal steam iron works fine, just make sure the setting is on low, and only iron the hem, no more. Test on scrap to make sure of heat range. You are only ironing no more than the seam allowance on your fabric. Keep the iron off the main body of the fabric...it can damage some coatings, but doesn't matter on the hem.
Try ironing your hems/seams before sewing. I think you'll be happier camper.