Better to take backup of primary database.
also check this
In our system, there is a physical standby database in data guard configuration. Is there still a need to do full backup and incremental backup?
Preferred to have full backup daily.
May be you can perform failover of your standby, in case there is no availability on primary database. Then do you think the standby system stability is same as primary and can give same performance?
standby location can be too far and this is only if in case of disaster.
Another case, lets suppose your standby is behind than primary for 4-5 days due to some issues. At the same time your production crashed then you may have chance to loose 4-5 days fo data. So recommended to have full backup always from primary database.
If you are checking stability of standby database daily and able to check your data by opening properly and you wont prefer to RMAN backup, Then it is fine. But it is highly recommended to have RMAN backup. BTW, You can have RMAN full backup on standby, if you want to avoid resources to use on primary