The short answer is NOT REALLY, and be advised that there are a lot of opinions in the forums and around on the Internet. Most of those opinions arise from the behavior and limitations on materials in nature, when a single source of light hits a surface and triggers all components. That light can be reflected, diffused, transmitted and so on, and hence these portions should not add up to a value larger than one (100%), otherwise the object re-radiates more light than it receives. It could be less, to represent absorption in the material.
However, the various portions of a Poser material respond to quite different – and quite independent – sources of light. Diffuse responds to direct and indirect diffuse lighting, Specular responds to direct specular lighting only, Reflection and Refraction responds to any (indirect) light re-emitted by any other objects in the scene – but not direct light sources themselves, and Ambient / Translucency represent some lighting or glow emitted by the object surface itself. See the next article for more details and background on this subject.
This brings up a different issue. In nature, adding up multiple light sources will increase the brightness of the surface, and my eyes will adapt to that (by reducing the pupil width). Poser rendering however is not capable of doing that (yet). This may cause overlighting on the image.
Hence, there are no limitations in balancing the various portions of a single Poser material, even when representing a ‘photo real’ surface. Choosing any values too high however can make an object overlit, or give it a very unnatural appearance.
Not for a “conservation of energy” concept or alike, but for a practical reason on the prevention of overlighting, there is nothing to stop me from using somewhat realistic values for Diffuse (80% to 60% will do), for Reflection (1% for organics, 4% for liquids to 10% for glass-like stuff will do, except for metals which may go 60%-98% but hardly Diffuse as a consequence) and similar values for Specularity as well.