I tossed the juice bag from the Riesling, but then thought I should go ahead and send an email to Winexpert to see if the precipitate is a bad sign, or no big deal.  I have to say, I’ve been very happy with their customer service – they answered me the next day!  Linda from Winexpert wrote that there are often brown pigments that settle out of the juice, and it’s OK to add in, as they won’t affect the finished wine.  Good to know!