My very first quilting book was written by Harriet Hartgrave...a purist and true traditionalist in the world of quilting. For years I did not use steam as I was terribly concerned with distortion after reading her books...now adays...I use steam.
I really don't think it can cause a lot of distortion unless you are working with curved seams or biased edges...or long seams (read - sewing your rows together). I say...use your best judgment depending on what you're working with.
(...and for the record, from my own personal experience I find that distortion is more a problem for fabrics that are not prewashed/preshrunk...)