The calculation of the 2-D polynomials and the location of the data points upon which they are based are CPU intensive tasks that should not be performed unless it is really needed. The best way of determining whether or not they are needed is of course visual inspection of the arc frame. If you are unable to detect systematic distortion visually then there is little chance that it will impact severely upon the reduced spectrum. If reassurance is required then the top or bottom order in the frame should be 2-D fitted and the View (V) option selected from the clipping menu. This shows a highly exaggerated view of the fitted points and any systematic distortion will be immediately noticeable. A typical interactive plot appears below.
Martin Clayton