If I may interject.
In the sutta references, by beeblebrox, ancientbuddhism and kenshou, above anatta comes after contemplation of anicca. To paraphrase one: "the aggregates are anicca, what is anicca is dukkha and what is anicca and dukha is anatta".
"attending to the perception of inconstantcy the perception of anatta will be established"
In the above case the perception of anatta is seen to be born from the proper contemplation of the other characteristics. This is of course fine and proper, and the perception of anatta that results this way is the fruit of insight and dispassion toward the aggregates.
However, the contemplation of anatta directly seems to be taking the stick from the wrong end. Doing such it might seem unclear just WHY something is not self. And if the proper answer, anicca, has not been well penetrated before hand then this contemplation of anatta amounts to an assertion of fact in absence of actual knowledge. Instead it becomes an view that denies such and such to be true, itself being unproven. And this kind of specious view making is not conducive to release, so far as I can tell.
So is the intentional labeling "Not self", "Not self", "Not self" a good practice in itself? I don't think it is, as such a label may be an unintentional reinforcement of a ontological position unproven by the persons experience. Whereas the contemplation of anicca and dukkha in the aggregates naturally leads to dispassion for all views of self: one no longer feels compelled by craving to identify with any of them as a self.
So, dispassion toward the aggregates leading to non identification with them as 'self' vs. an unfounded assertion that 'there is no self'.
I hope the distinction is not to subtle.
EDIT: made sure I said what I meant. Lol.