Often times a problem may require the assumption that we know the answer and work backwards, a sort of backwards problem solving. This type of problem solving may fall under the tag problem solving itself, but I want to know if it could/should become its own personal tag.
Tags should relate to the mathematical content of a post and the notion of "reverse engineering" is simply too broad and ill defined. One could fairly call the question "how can I find a polynomial with given roots?" and the question "how can I construct a CW complex with a given fundamental group?" as both examples of reversing some other process, but I can see no good reason to lump the two under some larger category (whereas tags like polynomials and cw-complexes describe the questions well). Moreover, "how can I find the roots given a polynomial?" is simply reversing distribution, so really, we could tag any question with this.
The problem with the tag really boils down to this: the notion of reversing a process may often be the motivation for a question, but it doesn't really affect the mathematical content of the question. Information like this has its place in the body of a question, but not in the tags.