I've never heard of that logic and I've only seen a few ramps which match the same speed of the highway with which one is exiting. 

With that being said, I do give you this http://onlinemanuals.txdot.gov/TxDOTOnlineManuals/TxDOTManuals/rdw/ramps_and_direct_connections.htm as law when it comes to editing in Texas in regards to exit/entrance ramps. 