Not necessarily either. It's not particularly hard to create a vector where in-order addition is the most accurate way to sum its terms. All you need is a sequence where the next term is close to the sum of all prior ones.
There just isn't a one-size-fit-all solution to be had here.
I think there is always a correct answer and the compiler shouldn’t make changes at least by default that are wrong.
But ways for the programmer to express his intent more clearly are always welcome.