sounds like very nice curtains and valance, in my thought process I would sew the tassels by hand after the valance was sewn, that way when I needed to launder the fabric, the tassels can be removed and resewn later,, Rene

If it were me, i would sew a clear button to the backside(lining side) after you have the valance all sewn together.That way you can just loop the tassel over the button,and when you have to wash it,just remove the tassel.This is the way i do my table runners that i put tassels on.
Or your button can match the lining fabric.
Kathi

Thanks for the suggestions. I had not thought of using a button.