You will probably get several different answers - all of which are valid...

I prefer rebuilding indexes prior to performing a backup. But I make sure that if that process fails it does not prevent the backup from being performed.

Others will prefer to follow the backup and rebuild indexes...

And yet, others prefer separating these out as completely separate operations. That is, a job to rebuild indexes could be scheduled to run at 1am and the backups scheduled to run at the same time - or completely different times with no dependencies.

I use this last option when I can no longer meet the maintenance window.