I hold the view that IDL has really superceded IBL. That's not to say that you can't experiment using both together, if you have something particular in mind, and understand the limitations of using both together. Otherwise, like hborre, I would not advise it.

When setting up a scene I do fast test renders with IDL off and with a desaturated HDRI IBL light switched on. When I get to the stage of IDL lighting setup, the IBL light gets deleted. (Some say it can slow down renders even when turned off - I have not yet tested this).
 
Also note that all light-based AO is automatically disabled when using IDL.
Material-based AO will only work if you enable it in each AO node in the Advanced Material Room.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)