When should I log in a plugin?
A few things to keep in mind.


LogLevel::Infois only going to be logged when devMode is enabled, so any overhead from excessive logging won't be an issue in production.
Excessive logging will consume extra memory because all logging information on a given request is stored in memory and flushed to disk at the end of the request, but overall I imaging that the performance hit will be negligible.
Excessive logging will cause your log files to rotate out quicker.  By default Craft will cap each log file at 1MB worth of data and will start to rotate out the older files after 6MB worth of data has been hit.