Hey,
We had used both the scenario's in my earlier projects:
scenario 1: We had too much of performance issue at HANA side and we didn't want to overload the HANA server again with these operations, hence we moved these calculations to BODS.
Scenario 2: Client was very specific on the BODS jobs completion time, and to improve the overall bods jobs performance we did the calculation on the HANA side and BODS had plain one to one mapping.
Its the call you need to take based on Hana Server Load vs Bods job Performance.
(My suggestion: As Hana is operation is expense and if minor delay is accepted, its better to do it on BODS)
Regards,
Deepak