Do we need a reception desk for every building?
No, you should not need a reception desk per area. Each reception desk has a maximum queue length, so maybe build one or two in the same building next to each other in the harder levels.
It is best to have your reception desk near your GP offices since the receptionist will direct people there first.
This is may be a better explaination: http://www.eudoxus.demon.co.uk/thc/hospital.htm#receptionist