If you mean: when to use public accessor methods instead of making the internal, private variable public my answer is "always" unless there is a severe performance reason.
If you mean, call your own get and set methods vs direct access to the vars w/in your class I still say call your own access methods. This way, any conversion, edits or rules you implement as part of get/set get invoked automatically by your own internal calls as well as external callers.
In pure OO languages (for example, Smalltalk) there is no concept of public - all internal vars are private and so you must use accessors. In less pure OO languages, you can make things public - however exposing the internals of your data structures and implementation is an exceptionally bad idea for stability and maintenance in the long run. Look up "tight coupling" for more on this.
Simply put, if you expose internal vars publicly, people can access them directly and if you ever change name or type everything down the line breaks. This is called side effects.
