As a general rule, more DNA you have and better it is. you will get a better representation of your sample and complexity of your library. which means less PCR duplicates, less bias and more peaks called.
However, sometimes you have no choice to start with couple of thousand cells and it's hard to get more than 1ng. especially with transcription Factor ChIP.
Diagenode is a global leader in Epigenetics and we do ChIP every day with little amount that we can't even measure the concentration after ChIP. For that we use our MicroPlex technology that allows to start from pg amounts of DNA.