see this commented by one researcher:

topographic phase removal is only performed if you need to look for deformation over time (DInSAR), else, skip that step and follow the procedures in graph builder in SNAP toolbox. Given 12 days minimum interval of sentinel-1a, we have to assume that there's no deformation taking place within that period of time. If the period of your study has indeed undergo deformation, you might need to get the DEM before the deformation took place.

Else, if the time issue is important for your study, maybe you can produce both DEM and deformation, then compare their differences. Anyway, sentinel-1a dem is not easy to produce due to temporal decorrelation unless your test site has high coherency (undisturbed/permanent ground features like mountain etc.). The constellation of sentinel-1b is coming so it will probably greatly enhance the

coherency  (see link)

http://www.esa.int/Our_Activities/O...l-1_satellites_combine_radar_vision

Sentinel-1 satellites combine radar visionThe twin Sentinel-1 satellites have – for the first time – combined to show their capability for revealing even small deformations in Earth’s surface.

What is the Procedure to derive DEM from sentinel 1A data using snap sentinel processor? - ResearchGate. Available from: https://www.researchgate.net/post/What_is_the_Procedure_to_derive_DEM_from_sentinel_1A_data_using_snap_sentinel_processor [accessed Dec 7, 2016].