acquire vs acquire_release in CAS

I have been working with C++11 memory order:

Many documents suggest to use memory_order_acq_rel with Read Modify Write operations. Does this mean every time we use atomic_compare_and_exchange, we should use memory_order_acq_rel?? Since atomic_compare_and_exchange is a read modify write operation.

But in many example I have see people using only memory_order_rel.

Can some one please shed some light on this...