Texts are all over the place about this.
For instance, one school of thought criticizes our tendency to conceptualize our experience, at the expense of actually experiencing the thing -- and says this is us sleep-walking through life (the dream is the conceptual template we overlay on experience). For instance, I encounter a tree, think "this is a tree" and never actually see the thing before me. This would seem to imply that labeling during mindfulness is a repetition of this mistake.
Another school of thought sees the labeling as a way to step outside of the experience, to not get sucked into it.
So who is right?
The issue here is not getting pulled in by attachment and habitual reactions. While these are a function of conceptualization, they are a function of a specific type of conceptualization.
For instance, if someone does something I dislike, and I conceptualize this person as a "jerk", I likely have a strong reaction to this, and am falling into the same attachment (aversions) that have caused me so much pain.
On the other hand, if I experience a reaction of aversion towards the way this person treated me, but start labeling the components my experience as Skhandas, then I'm effectively deconstructing the experience and undermining attachment in the process.
On the other other hand, if I watch the experience as it is, then I can be with the sensations without the problematic concepts.
So really, there is no right or wrong answer; the key is whether your technique takes you out of the conceptual trap that keeps you attached to the phenomena or reinforces it. In this case, it's about fighting fire with fire (concepts with concepts) or fire with water (concepts with bare sensations).
The upshot is that you don't have the luxury of treating anyone as an authority, but have to see how you respond, and decide on the most effective method.