Is it generally safe to use a lower wattage bulb in a light fixture? (Specifically Halogen bulbs)
AFAIK, bulbs with a GU-10 base are driven directly from electrical mains, in which case it's OK to use a lower-power bulb.  Your fixture won't reach the temperatures that it's capable of withstanding.

The only place that I know of that it might not be OK is with low-voltage (usually 12V or 24V) lighting systems such as track or task lighting: some transformers have a minimum power output that they like to supply, so if you use lower wattage bulbs, you should use more of them to compensate (somewhat defeating the purpose).  If this applies, you would have an entry for Minimum power on the specifications for the lighting system.