Update from May 2020:
The below is only true for Blender Version 2.79 and below. The current version (2.8x and up) manages tile size automatically, so this addon is no longer useful or used. The new major release 2.8x has been available since July 2019 and is a replacement for all earlier releases.
To answer your question: Yes, it is accurate.
I have been using it for years now and it guarantees optimal performance. Especially when switching from CPU to GPU, the tiles size is always wrong, as I will forget to change it and render times suffer horribly (GPU renders super slow with a small tile size, CPU lags with a large one)
I wish this was turned on by default, in years, I have not found one drawback to it. I have always heard people tried to find a good reason why it is not on by default, assuming firmly that there must be one. It can't just be optimal and automatic at the same time, right? But so far, nobody could ever show me a concrete reproducible negative effect of it.
Well, Auto Tile Size does have a drawback: It is not activated by default ;-)
It should be, at the very least, activated as a addon, to show up in the render performance settings for the user to notice, that yes, there is an automatic way.