When deciding what areas to air condition you need to balance cost against convenience. The more you air condition, the higher the cost of the air conditioner and the higher the running cost. The running cost should not be ignored as more and more cities are likely to have higher summer electricity tariffs, to offset the increased air conditioning load. This page mainly considers ducted reverse cycle systems, but some of the information is relevant to other types.
Most people only reverse cycle air condition the main rooms of the house (Lounge, dining, kitchen, family room, bedrooms, study). This is because you rarely spend much time in rooms like laundries, bathrooms, etc. In addition if the house is air conditioned these rooms will be cooler in summer (and warmer in winter) than they will be in an un-airconditioned house. Another reason for not air conditioning bathrooms, is that the air motion can make you feel too cold.
As people rarely use the whole of a house simultaneously, it is generally a good idea to allow to switch unused rooms off. This is called zoning. Generally it is necessary to treat any area that doesn't have interior doors as a single zone. (see diagram). So generally an open plan kitchen, dining, family room is a single zone. As the whole of a zone needs to be cooled at once, the air conditioner you buy must be capable of cooling and heating the largest zone.
Think about whether there are going to be any extra rooms that you must have on at the same time as the largest zone. For example if you work from home, your home office would need to be cooled at the same time as the main zone. If this is the case you will need to add this load to the main zone before selecting the air conditioner. However don't go overboard, because most of the time the reverse cycle air conditioner can cool (or heat) much more than the design area. This means that for most of the time you could air condition the children's bedrooms when they are in their rooms. You will only need to switch them off for a few hours on the hottest of days. During this time everyone can congregate in the main zone.
Another item to think about is the flow of return air. Generally if you have a central corridor, the return air grille will be placed in that corridor, so that air can get back from any of the air conditioned rooms to the air conditioner itself. Even though the corridor or hall may not have a supply grille to it, the heat that it absorbs has to be added to the air conditioner load. Generally this is not a big deal, but it can be a problem for houses with a grand hall, with large windows or skylights.
If you intend to cool more than the main zone, I suggest you work out the load for each zone separately, and then add them together in different combinations to see what areas a given size air conditioner will be able to do on a hot day.