I've been on tours in Morrocco a few times ( I currently live near Algeciras ), and the tour guides earned their tips, through not only showing us the places to see, but protecting us from the local "salespeople", and arranging camel rides. I spent about £15 - £20 ( probably about US$25 - US$35 ) on tips that day.
On the other hand, I've been on tours, where the tourguide, simply pointed out the various things, without showing much interest in what he was doing, and I felt like I had already paid( in buying the ticket ) for the service what I received.
On this basis, I don't think you can really say how much(or if) a tourguide should be tipped, until the tour is over. Being pushy for tips, is IMO, bad manners, and bad manners is bad service.