I would say that you should never need to access any interface signals in your sequence. The reason for this is that your sequence should generate stimulus independent of any DUT or other interface signals.
The only place where you should access interfaces is in your agent drivers and monitors. It is the job of the drivers to drive signals per the protocol and provided sequence items. It is the job of the monitors to watch the interface and provide sequence items back to the scoreboards, as well as provide interface status to other components.
Perhaps if you explain what you are trying to accomplish, we can provide some more specific guidance on how to achieve what you desire.