I think it's important to get the sizing out of fabric before washing it. First of all, I can't stand the smell of the sizing, nor the feel of it. You can't be certain that the threads of the fabric have been aligned properly by the manufacturer. Also, it's really important to separate colours-I've avoided real disasters that way.Washing, then stretching the fabric if needed will help your project. When drying the fabric, take it from the dryer before it's bone dry--then you can adjust it as necessary. I press the fabric using a light spray starch. For newbies, there's a difference between pressing and ironing. Pressing is a light, straight up and down motion-not swirling the way you'd press a shirt. This prevents distortion of all your hard work!
I hope this helps.