While it is perhaps not critical that the same breathing technique be used for each control question, there may be an advantage to using only one or two such techniques (possibly in combination, e.g. breathing more deeply and more rapidly). By limiting the number of techniques used, the polygrapher may infer that the recurring reaction(s) he sees in the charts are your "typical deception response," rather than puzzling over a different pneumo response for each "control" question.