Technically, you may not need to latch any of your cabinets as the cabinets themselves are typically not very dangerous. It is the contents of the cabinets of which you need to be concerned. Ideally, hazardous items should be stored in locked upper cabinets. Practically, this is not feasible for most families, so latching of most or all of the cabinets (doors and drawers) below counter height is typically advised.
Most parents are well aware of the need to lock up the poisons and cleaning supplies, but don’t stop there. In the kitchen, knives, cooking utensils, trash, plastic bags (new & recycled), spices and breakables should all be secured. Medications, cosmetics, jewelry, toothpaste, etc. should all be secured in the bathrooms. Elsewhere in the house, don’t forget to latch the china cabinet, liquor storage and entertainment centers. Although safety is the primary motivator, parents may also want to latch additional cabinets from a housekeeping perspective.
Traditionally, parents have allowed children to play with pots and pans. The danger of this approach arises when children grow to perceive pots and pans as toys. A child may subsequently reach for a pot of boiling water on the stove since she perceives it to be her "toy".
To limit the number of latches required, you might consider consolidating your hazardous items into a fewer number of cabinets and then latching those cabinets accordingly.
When you latch is directly tied to which type of latch you use. The simpler the latch, the sooner it needs to get installed. Ideally, latches should be installed before your child has explored the cabinets. If not, many children will make it their number one priority to defeat some of the simpler latches.
There are a multitude of cabinet latches available on the market. Unfortunately, not all of them are well made and of those that are, not every latch will work on every style of cabinet. KidCo makes a good latch that is very versatile and provides a moderate level of deterrence. SaferBaby makes a unique side mount latch that is very effective on hard to latch drawers. The best latch available is the magnetic Tot-Lok (note: these are now widely available since Safety First purchased the distribution rights). Tot-Loks provide the most child resistance by far. Although somewhat difficult to install and considerably more expensive, the level of safety provided and the length of effectiveness make them a terrific value.
So, in answer to your question, no, you don’t have to latch every cabinet. You should, however, consolidate the "bad stuff" as much as possible, select your latches carefully and install them sooner rather than later.