What should I set for flashback timestamp timezone property?

 I need to run my database in Greek Time. We have already set the system clock to Greek Time. 

what should be the value for DBTIMEZONE and FLASHBACK TIMESTAMP TIMEZONE. 

We are only using DATE and TIMESTAMP data types. But,  we may need to use TIMESTAMP WITH  TIMEZONE datatypes. Currently, we are not enabling the FLASHBACK DATABASE option. But, we want to make sure that the timezones are properly setup so that we can use these features in future.

I would also like to understand whether I have to use the timezone name to take advantage of the day light saving.