You should never power off to introduce new drives, UNLESS you are migrating an entire VD from another machine or controller.
I have a Dell R710 poweredge with a PERC 6i controller. Currently there are 3 drives installed as RAID 5 and are indicated physical disks 0-2, all are online with no hot spare. My question is:  I have 3 more drives to install in the server, exact same drives that are in the current configuration. Do I have to power down the server to install these drives?  (I am going to use OMSA to expand the existing single VD that is configured.  I saw a post that said don't power down the server, so I'm just wanting to ensure that all I should do is go to the server, install the new drives in the open slots while its powered up and then perform my other functions in OMSA to expand the RAID. Thanks. 
Power down when inserting new hard drive (additional)