There are no risks. Media server AND clients CAN be at a higher triple dot digit version than the Master server.
They CANNOT be at a single dot or double dot digit higher version than the master server however.
So you can have a Master server at 7.5 and a Media server at 7.5.0.2 and a client at 7.5.0.7 and another client at 6.5 for example. (though note version 6.x is no longer Supported by Symantec since over 2 years ago)
A Master server at 7.6.0.3 and a Media server at 7.6.0.4 (this is fine), however you CANNOT have a client or another media server at 7.6.1 as that is a double digit version higher.
As long as the Master is higher or the same double digit version than the media servers
and media servers are higher or same double digit version as the client, it is supported
As for other risks, if you have your catalog backup configured, ensure you keep your DR files on a filesystem that is not hosted on your master server. If the disks blow up, the server goes down AND you lose your DR file. Also ensure it is e-mailed out.
Ensure you catalog backups are to tape or at least duplicated to physical ejectable media for safe keeping. Ensure you do a catalog backup every day if possible - that ensures you never lose too much backed up data in the event a DR recovery is needed.
As for other Risks - there are too many scenarios to cover. IF you have catalog backups set-up and running successfully you should be fairly safe.
Read the 7.6 documentation for more info. The Admin and Troubleshooting guides are good for this type of question. The Troubleshooting guide here has some good Disaster Recovery info on page 204
7.6 Documentation: http://www.symantec.com/docs/DOC6488
Troubleshooting: http://www.symantec.com/docs/DOC6470
