I think that is generally regarded as very bad advice, unless you're at a specialty bra store that probably isn't in a mall.

The "professional fitter" will helpfully fit you into whatever sizes they carry, and not all brands are the same. Cup size and shape vary a lot.

As the more upvoted comments above, the subreddit /r/abrathatfits is the place to start - most horror stories start as "I went to Random Department Store/Victoria Secret/etc. and they told me X"

Moreover, if you do any sort of clothes shopping, the individual variance between items of the exact same size/style/color is very wide.

For pants, inseam and waist sizes that are literally two of the same pants grabbed off the shelf, have more than a 1" disparity between them.

Also true for bras, even if the size is the same.

And even worse than that is comparing sizes across brands - 34x30 is not the same with every brand, or every pair of pants, and a 36B isn't a 36B everywhere.