As recommended by RAM Development team, we are starting the process of taking the regular backups of good indexers so that in case the indexers will get corrupt some time in future, we can restore the backups.
Can we take the backups even at the time when the indexer was building?
OR, what all things we should take care of while taking the backups of index folder?

You can not take a backup while the index is running. You must follow these steps.

To backup
This must be repeated on each node in the cluster
1) Stop all the servers on a node
2) Backup the index folder. It's location can be found on the configuration page.

At any point, you can restore this backup by following these instructions
This must be repeated on each node in the cluster
1) Stop all the servers on a node
2) Delete the existing index folder. It's location can be found on the configuration page.
3) Restore the backup folder to the same location
4) Start the servers on the node. The index will automatically start building the delta (time between the backup date and current time)

PS:

When finding index location from configuration page, remember that the
configuration page actually points to the PARENT folder that contains
the com.ibm.ram.index folder. So don't backup/delete the PARENT folder
that is shown in the configuration page. Backup/delete the
com.ibm.ram.index folder instead. When restoring, restore the
com.ibm.ram.index folder itself into the parent folder.

Rich