Garden nest boxes provide a tremendous source of pleasure and interest and are an ideal tool for studying birds. Contrary to popular belief, most birds have little or no sense of smell, so there is no need to fear that they will smell that you have visited the nest in their absence. Most species are fairly tolerant of a gentle human approach.
During their breeding period however birds are sensitive to disturbance and are at their most vulnerable. Avoid situations where your presence might affect the natural behaviour of a nesting bird eg. if your presence is keeping a parent off its eggs, you should rather move away.
Once the eggs have hatched, nestlings and fledglings are best observed from a distance as close inspection may result in their premature departure.