When my parents applied for their visa, the counselor didn't even look at the invitation letter. The only thing that matter is the "Strong ties to the home country". They must be able to provide enough evidence showing that they do not have any intension of immigrating to US. Lots of parents that came to US to attend graduation stayed here illegally.

The bottom line is show as many sources of income your parents have and property such as lands, vehicle, bonds and stocks, banks etc.

Hope that helps

Uhi Nepali