> It is much easier to do it at design time

Is it? I always worked the second way (starting from seq_cst and then when the core design matured enough and didn't change for a few months, trying to see what could actually be relaxed). I'd be very afraid that in the first case, you start with say relaxed semantics somewhere, them you change the design because the requirements changed, and now you have to go again through all the atomic operations to make sure the assumptions all still hold.