I don't believe they throttle the engines up or down much during the second stage burn. Fuel decreases ~linearly and thrust is relatively constant. Acceleration increases as fuel mass decreases.
I would be surprised if that was the case, my reasoning to that is that computing where a thing is going, when it's under going with changing acceleration AND changing mass, is pretty complicated. Especially if you already have the capability to throttle the engines and keep 'a' constant.
They might, I'm not saying your wrong, I'm just saying that I cannot imagine how you would justify the added complexity of doing it that way.
Don’t they throttle back at MaxQ?