Make different builds on your STF character and play till you learn what is your favorite class you'd like to play for a long time. Then make your main the class you want and don't retire/respec anymore.
Or, if you want to get max generation xp bonus -
Just play till 31 lvl, retire, then make different build, retire again, etc, until you're 16 gen. At that time you would probably know what's your favorite class, so you wouldn't need to respec anymore.