Are you talking interior as well or just exterior? Exterior I'd go with conduit, that way the home owner can paint over it so it blends. Junction boxes at the ends to mount the cameras to depending on if they are installing dome or turret. If the wires will go thru the boxes and into the interior, be sure to seal that hole with a heavy duty silicon putty or caulk.
Bullet cameras can be mounted directly onto the exterior surface if you don't want to use junction boxes but remember the connection needs to be weather tight.
If there isn't a good solid surface and siding is the only option, you can always try and pull away the siding from the house and when you drill thru, put a small piece of wood behind on the other side so when you drill thru that it secures a bit. That's how I mounted most of my turrets in my roof soffit. Wasn't comfortable using the screws and mounts so I pulled the soffit off and drilled right up thru it and into the wood. Three screws meant I used 2 pieces of wood, one about 4 inches long to cover two of the holes and the third hole I used just a 1 inch piece of wood. Secured it very nicely.
