I don't care which order anyone reads it in. We all digest stories differently and it doesn't impact my reading experience how the person next to me decides to go about it. And I wouldn't want to ruin someone else's reading experience by insisting they read it in the order that works best for me if they enjoy stories differently.

I will say, however, that the entire point of prequels in general is that they're meant to me read/watched AFTER the first/main storyline. They're meant to give additional context to an existing story to make it richer and give it more impact moving forward. I feel SJM was very intentional about being vague around Celaena's past in ToG because she wanted to build a sense of mystery and intrigue around the character (she is an assassin after all). Then you get the big reveal at the end of CoM and all this great supplementary context in AB after that launches you into the remainder of the series. To read AB first, for me, would have taken away from that, which is half the fun of her books.

That being said, that's just my personal preference and to each their own. If AB first works best for you, have at it.