Currently, many of these questions are tagged with the very generic calculus, and some of them with integration, summation, or similar tags. (Admittedly, some of these will be off better under just the integration tag.)
Given the volume of these questions (and the fact that there is no easy other applicable tag other than calculus) there seems to be enough justification for introducing this synonymous pair of tags.
NB. I am aware of the subtle difference between Darboux (a.k.a upper and lower) sums and Riemann sums (which may take intermediary values as well) but the distinction is not big enough to merit separation -- not to mention the ubiquitous confusion between the two.
Edit in reply to user79365's answer (it was far too long for a comment):
It seems that the origin of our disagreement lies in our perceptions of the "help in searching" (this tag will admittedly not be marked as favorite by many) it will have.
I contend that the words "Riemann sum" are at best likely to occur in the questions I imagine under this tag, and since "Darboux sum", "upper sum" and "lower sum" are all different terms closely tied together, one is likely to miss out on search results when restricting to only one of them (or, for that matter, by combining them all).
Moreover, there are instances of Riemann sums being computed before the word "integral" is introduced, questions about the existence partition refinements with certain properties, and sums that give rise to an obvious "Riemann sum interpretation". A tag to unify them all still makes sense to me, because it ranges over a considerable array of currently existing tags.
This all in addition to the fact that tags have an enormous impact on the "Related" list, and as such make the localization and subsequent identification of duplicates a lot easier (when used properly). At present, there seems to be no good way of locating duplicates to questions that would fit in this tag, and the trouble this has generated for me at certain times has certainly played its part in the creation of this proposal.