Mark,
You have a couple of options, depending on the onDemand flag in your config.
If its not specified or set to true (default), the add-in code isn't loaded until you interact with the control, in which case you can put the version check in the click event or even the add-in constructor.
If you have specified false then the add-in code is loaded when ArcMap starts. You could put the check in the constructor or in the OnUpdate override. Given that OnUpdate is called several times a second, its not an efficient place to put lengthy operations and is normally used for condition or state checks.
Setting OnDemand true in this case seems like an overkill so I would just go with the controls click event.
Where are you reading the version info from?