MS Cruises do a scheme where you pay one lump sum and it gets split between all the staff (including the ones you dont see). Other than that, the general rule is 'tip for service', but remember that taking your photo for you is a service, as are many other things.
The worst is being expected to tip for using dirty toilets. Clean ones got a bigger tip from me.
As for disrupting the economy, many of the people you encounter are dependent on tips, and their pay is based on the assumption that they will get tips.
The area around the Pyramids is the very worst. Every few metres someone asks for money.
One thing we had from our guide is that you should not give money to children, because it encourages them to skip school. If you want to give them something, then pens (even cheap bics) are much appreciated.