There’s probably no reason to have Wi-Fi enabled if your device is sitting there unused – and your battery life will be improved if your tablet isn’t constantly syncing in the background anyway.
Note: Obviously if you have a 3G or 4G device, disabling Wi-Fi will increase data usage and probably won’t save you any battery life unless you’ve configured your device to not sync on your mobile connection. This does work really well for a Wi-Fi only tablet though.
First head to Wi-Fi settings, pull up the Menu, and choose Advanced. Then use the “Keep Wi-Fi on during sleep” option.
You can choose between Always, Only when plugged in, and Never.
Once you’ve selected this option, when the tablet goes into sleep mode, the Wi-Fi will be cut off. It’s worth noting that it doesn’t cut off immediately.
You can head to the Data Usage section of Settings to change the other options for mobile network data.