If your class has getters and setters for fields, you should be consistent throughout the implementation of that class in whether you use getters and setters or access the fields directly. It doesn't generally make sense to mix levels of abstraction. Otherwise, how can you rationalize using them in some places but not in others? In addition, if there is ever a change in how the getters are implemented, you could end up with subtle bags. toString() is part of the implementation, so it should be consistent as well.
In terms of performance, it shouldn't matter - your trivial getters and setters should be final methods anyway, so they should be inlined anyway. IF they're not final, you should ask yourself why and probably avoid accessing the fields directly unless this is really what you mean to do.
