I'd always gone on A times before the 2-1 but found set times the only way to get through the 2-1 it helped my DS learn his new routine much better than never knowing when he was going for a nap. I think it's a really good idea. I'd be careful of being too set with BT simply because EBT can become the regular thing with the 2-1 until the nap begins to lengthen and gradually the EBT can return to BT. Even so, you can have an 'almost set' EBT/BT.
I think patience and low expectations are key in the 2-1.