Hi Rahul,
For phase 1 ISAKMP, these parameters exists on both devices, at the time of exchange between ASAs will pick a proposal given by an ASA, assuming AES-256 encryption is defined for the phase 1 negotiation. If you choose not to use AES-256 encryption for phase 1 negotiation, only way to force it by removing policy that contains the AES-256 encryption for phase 1.
Hope that answers your question.
Thanks
Rizwan Rafeek.