Depends on what you mean by "stretching" for calibration?  If your question relates to 18O abundance measurements by IRMS then the answer is yes; scale normalization of measured d18O value to the VSMOW scale is a muist.  The calibration equation is: scale calibrated d18O = stretch x d18O measured + off-set; stretch factor and off-set are determined from the comparison of measured with accepted d18O values for 2 reference materials of different d18O values.  The requirement for scale calibraton is due to a phenomenon called "scale compression" and the resulting need of scale calibration to achieve comparability of isotope abundance data between different laboratories.  Ideally this is achieved by contemporaneous analysis of matrix matched reference materials when analysing samples.  For example, when analysing organic compounds for 18O one would also analyse reference materials IAEA601 and IAEA602 to normalize measured d18O values on the VSMOW scale.  For 18O analysis of (bio)phosphate you could use the 18O standards reported by Halas et al. (36. S. Halas, G. Skrzypek, W. MEIER-AUGENSTEIN, A. Pelc, and H.F. Kemp (2011): “Inter-laboratory calibration of new silver orthophosphate comparison materials for stable oxygen isotope analysis of phosphates”; Rapid Commun Mass Spectrom., 25, 579-584; DOI: 10.1002/rcm.4892).

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