Using resistors at their rated power
You may encounter the term 'derating' in this situation. For example, a 250mW resistor built into a design so as to dissiplate 200mW is derated by 50mW. You typically leave at least a little margin because a resistor's actual resistance may depart from its stated value when it becomes too hot, and it will generally become appreciably heated if you operate it at its specified maximum power rating.

The amount of derating required in a design has everything to do with the physical arrangement of the circuit; its enclosure, whether there is air flow (or some other coolant), how close together the components are, and of course, the expected power dissipation of all the other parts of the circuit, too. This is commonly called a 'thermal analysis'.