I started to reply and explain that I receive a lot of cards and so I keep a couple of pretty ribbons strung up in the hall to display them, then it it occurred to me that I had forgotten to take down the Christmas cards from there! I've just done it now. 
It wouldn't bother me in the slightest if friends or family read my cards, but in order to do so they would have to take them off the line or tilt their heads sideways to read them. Before Christmas there were cards saying Thank You, Thinking of You, New Granddaughter etc. - all fairly innocuous and most say on the front what they're for. I would occasionally read cards in my children's homes and at my parents-in-law's home if there were a special occasion, especially if it was under discussion. So for example last year my father-in-law won a very prestigious award and received many cards that were put out on display. I can't honestly remember, but I assume I would have said, 'May I?' before reading them.
It really wouldn't be a big deal for me, but then I don't display cards that I wouldn't want read, cards from clients for example.