They can come here anytime/anywhere/any flight/ -----as long as they have valid visa.

 Will they not have problems in the port of entry? Will they not be asked, why are they coming after the graduation? They gave the visa for to attend graduation, don't you think that would be a question that my parents shall be asked?

 Anyone else want to share put light on the matter? 

IT is a tourist visa for multiple entry purpose. it does not matter. YOu can come to the state as many times as you want. You just need to comply with the number of days you can stay during your travel.

 Your parents are not refeiving a "Graduation VIsa" they are receiving a Tourist Visa that allows multiple re-entry. They can just state they have lots of places to visit here so had to come again.

 My mom got the VISA on my graduation. She didn't come on my graduation but has come to the US three times after that. They don't care what she got the visa for. She just can't stay here for longer than 6 months, it doesn't matter where she lands or anything. 
Take a chill pill. 

Doesn't really matter as long as your parents have valid VISA.
Related thread: http://sajha.com/sajha/html/index.cfm?threadid=79201
~GoodLuck