Firmware upgrades can fix issues with a given tv or provide new features that your tv didn't have before. However, firmware upgrades can also introduce new problems that didn't exist before. As such, I would recommend waiting at least two weeks before applying a new firmware, just to be sure that are no problems with it.
Regarding viruses, I guess it is technically possible to have a virus for your tv. That said, viruses need to be written specifically for any given target. In the case of computers, it usually is the operating system, browser, or application. Would a hacker want to write something to specifically target a tv? Probably not.