Geometric adjustement

Hi everyone. I would like to compare different time acquisition S1 imagery and it is known that many types of geometric correction procedures are allowed in SNAP. About that, got next questions:

Different time imagery must be co-registered in order to make reliable multi temporal change detection? or

Is it enough to apply geometric correction for each imagery and then perform every needed procedure for multi-temporal analysis?

What about if it is necessary to perform different Sentinel imagery (for instance, S1 and S2) multi – temporal comparisons?

Thanks in advance.