It's always sensible to consider the risk of consumer devices exfiltrating sensitive info from other machines on your network; or, in a worst-case, providing a route into your network for an active attacker (though the latter normally requires the attacker to work for, or infiltrate, the device's manufacturer, so that's a fairly long shot).
However, be proportionate: is there actually data freely accessible on your network that's sensitive enough to need protection?
Let's say the only other devices on your network are the router, a PC and maybe a smartphone or two. These have their own firewalls, and only allow communication from other devices in ways laid down by the manufacturer/operating-system vendor, and normally subject to the user's control.
Where you do set them up to allow access (e.g. for Windows file and printer sharing), you can do this in a controlled manner, requiring credentials for access. If you want the TV to access media stored on your PC, you can tell the PC to share only the folder that holds those media files.
If there is no such sharing in place, your TV will only be able to see some basic metadata about your network: what type of other machines are on it, at what times, and so on.
If you can't be confident that your other devices are adequately protected against undesired access, and they hold data that is sensitive enough that you absolutely cannot take the risk of compromise, then using the guest-network is a simple remedy.
If you still require some communication between the TV and the network, you can give your router the job of controlling which specific devices are allowed to contact each other. This level of control isn't always available with consumer wifi routers, but a replacement can be had quite cheaply: either buy second-hand (although bear in mind the device could be compromised), or get one of the cheap consumer models that can be modified to run openwrt or similar highly-configurable open-source firmware.