I press as I go along and unless it has been wadded up and thrown in a tote some where I don't have to press the whole thing before quilting. I quilt it then wash and dry it. I wouldn't DREAM of ironging after its washed and dried.
HOWEVER...I know there are people who DON'T like the wrinkled/old look of quilts therefore they prewash/dry and they do everything humanly possible NOT to have the wrinkles in their quilts. So I am sure this would include ironing their finished quilt. The only way I could see it "ruining" a quilt is if you used monofilament thread to quilt with, the iron would melt it.