Frequently when performing data preparation or analysis in GIS the spatial reference or projections need to be modified to match. Also, much of the time the data needs to be modified to match one specific feature class. The difficulty lies in frequently also needing to select a transformation if the geographic coordinate systems do not match. In the Gist included below, if this is the case, the first transformation recommended by ArcGIS is used. In this way, no matter what, the data is guaranteed to be in the right spatial reference for further processing.
Hopefully this makes your life easier, and save you a few headaches!