Is it safe to delete the contents of Library/Application Support/MobileSync/Backup if using iCloud?
Yes, it is safe to delete it, if you are sure you want to lose those backups.

To do so, in a more elegant way, just:


Open iTunes Preferences 
Click Devices
Locate the backup for the iOS device you want to delete, select it, and click Delete Backup.
Confirm you wish to remove the selected backup by clicking Delete Backup.
Click OK to close the iTunes Preferences Window.


Extracted from Apple's support document about iOs backups.

Confirmed: Deleting through iTunes will delete all the backUp files of the selected device.