I think there's a SBSettings toggle for user agent faker. You can also set an activator action to quickly enable and disable this toggle. But I can't remember if you need to kill the app and then reopen it for the changes to take effect. I dont know of any tweak or app that let's you whitelist or blacklist specific sites.
I use the dolphin browser which has the option in settings to turn on desktop mode. When it's on, every single website will load the desktop version. If I go to a site for which I prefer the mobile version, I just tap on settings then tap on desktop mode to turn it off then reload the site. That's what I do. Kind of a pain but not a deal breaker for me since I rarely prefer the mobile version of a site. But I would be interested to find a tweak that allows you to choose certain sites to always load the mobile version without having to disable the universal desktop mode.
