I drink half/half tea. Half sweetened, half unsweetened. I think it tastes better when I put sweetened in the bottom of the glass. A friend says that's silly. It all mixes together. Is my imagination making me think it tastes better when sweet goes in first?
The answer of the question has little to do with cooking, and more with knowing how human cognition (= how do we know that we like something) works. To your brain, "X makes me think it tastes better" is the same as "it tastes better", no matter if X is the chemical composition of the drink or the knowledge that the sweetener went there first, or something entirely else.
It is possible that there is some objectively measurable difference in the drink. It is also possible that there is no such difference. In both cases, the outcome is the same - you have some reasons to like the drink sweet first better. Dividing them into "objectively measurable properties of the drink itself" and "anything else" is irrelevant to the outcome (your taste preferences). It specifically does not mean that, if your reasons turn out to be "anything else", your liking is somehow less real, or that you are being silly.
So, the answer is basically tautological. If for you it matters that the sweetened goes in first, then for you it matters that the sweetened goes in first. And it has nothing to do with whether if matters for your friend or not, whether you could taste the difference in a blind test or not, or any other possible tests of the "reality" of your feeling of liking.
Pretty minor but sugar tea is more dense so pouring it on top will be a little faster mixing. But just a quick stir will mix the sweetened and unsweetened tea. No stir is not as fast of mixing as you might guess.
You might be using a different ratio when you pour sweetened first.