Thank you all for your comments. I agree it should be OK. But what if the process to produce that bargain de-ionized water (see my OP) has exchanged the ions that the steam iron doesn't like (Ca, Na) for other ions that offset the RedOx balance, for instance. Some picture-taking opportunities happen only once...
So, I'll buy one, and do an A/B comparison (water "x" versus usual one) with HC110 dilution H, which, being high dilution, should be most sensitive to water quality.