How are you backing up your Index locations? If you are not using some form of snapshot technology you would want to put the environment in Backup mode then.
If you are using snapshots for everything then you should be ok doing the online backups. I will post this for your viewing pleasure: :)
Details:
To ensure an optimal running EV setup it is recommended to implement a backup procedure that includes cycling the EV services daily or regularly. Regularly restarting the EV services will help clear/reset MAPI resources that are in use or other resources that could not be cleared successfully.
1. Ensure that proper backup practices are followed and that at least the tasks (primarily the Agent tasks which comprise of Archiving, Retrieval, Journal, and Public Folder tasks) and Storage Services are all shut down as part of backup procedures. See the EV Administrator's Guide for further details.
2. As per Technote http://entsupport.symantec.com/docs/285130 , MAPI related issues can be avoided by restarting the services daily.
Enterprise Vault 8.0 and higher
With Enterprise Vault 8.0 and higher the Storage and Indexing services no longer need to be restarted when using the backup mode in Enterprise Vault 8.0. However the Task Controller service can still benefit from a restart to clear/reset MAPI resources that are in use.