There isn't a yes or no answer for this one. It's really up to you. If you wish to be contacted back, it might be a good idea to write your address on the postcard.

Users frequently ask why can't the senders address be sent automatically to the person who registers the postcard when he/she enters the postcard ID. The reason is that some people are not comfortable with this. If they choose not to send their address on the postcard, we won't do any different.

Note that when you register a postcard you can leave a message. The recipient can use this feature to request the sender's address if he or she would like to send a card back and the sender didn't include it on the postcard.