it is not strictly necessary.
however, if your first step is ImageCalibration, your CR2s will be read in, calibrated, and written out as XISF anyway.
another issue is that CR2 files can be *very* large and must be read into memory in their entirety. this can cause ImageIntegration to use a lot of memory. XISF files (as long as they were not saved with compression turned on) can be partially read from disk, so integrating uncompressed XISF files is easier on the computer's memory system.
rob