the only difference between IPP mem allocation functions and C run-time memory allocation functions is alignment. The IPP memory allocation functions do align allocated memory block on 32-byte boundary, whereas C run time memory allocation susally algins block on 4-byte boundary.
So, answer is yes, when you allocate images of the same size, IPP memory allocation functions will always align allocated memory block on 32-bytes boundary and so they will provide you the same step value.
Note, alignment depends on processor u-arch features, and is choosen to provide the fastest access to memory block (keeping allocation overhead within reasonable range). That mean alignment may differs on different processor generations, for example between IA32 and Itanium. That does not necessary implemented right now, but that is possible in future.