Database fields are always stored in UTC. The 'localize'-option has only influence on the UI of your application.
In general, always use Localize = No, when the time is not relevant. If the time is relevant in your application and your application is used solely in one timezone, you will not have much troubles. If you have an application where the time is relevant and is used in multiple timezones, you should follow a masterclass in how to deal with datetime attributes in Mendix, because it's a pickle ;-),