does conference presentation topic need approval from PI
That would depend on a few things, including your field. What is the custom there? If the PI is, or should be, a co-author then you would need it, ethically, I think. S/he might have other plans. If the work is partly theirs, then you need it in some places legally, if I interpret other answers on this site correctly, and ethically in any case. 

I'm depending on your use of the words "with a former PI" here. To me that implies they have some contribution to the work that requires acknowledgement, hence approval.  

But, for political purposes and keeping good relations for the future, you should probably seek it in all cases.