It depends on the goal of your study and the approach you’d like to follow, this issue is well explained by our colleague @ABraun in the following posts,
- Radiometric & Geometric Correction Workflow on the options of calibration
- Radiometric & Geometric Correction Workflow on Beta0, Sigma0 and Gamma0
- S1 radiometric correction on the effect of Terrain Flattening on images
It’s very easy to process each subswath individually and merge them later on,