I don't. My console is enclosed in a cabinet which means that it doesn't have much room for exhaust when it's not open. My Wii used to overheat for that reason during the night when it was in WiiConnect24 mode and wouldn't turn on until it cooled down. I turned off WiiConnect24 and never had the problem again. So, now I keep my Wii U completely off.
Here's a description of what Stand-by Mode is:
While in standby mode, the Wii U will:
Continue to download items currently downloading in Data Management, including content purchased from the Nintendo eShop, system updates, software updates, and add-on content Regularly connect to the Internet to check for available SpotPass data or software and system updates Install completed downloads, including system updates if have installed system menu version 5.0.0 U (released on June 2nd, 2014) The Wii U console will display an orange LED and power on any connected USB storage devices while performing these functions.
The standby functions will also work when the Wii U is in Wii Mode.
After what happened with the Wii, then I don't trust stand-by modes, so mine's always off when not in use. The Wii generated some heat when in stand-by but didn't have the cooling fans on, so over time the heat would eventually melt the GPU. I was one such unfortunate victim.