Well the way I play Janna is that I will start with shield and tend to try and max it at times and still throw out tornados in the beginning of the game and around mid to deal damage. That's the reason I get Athene's and zhonya's because I want to give her a little AP so that she can hurt and make it easier on the ADC to get a kill. I'm looking into going exhaust into clarity because by mid-game I don't need it anymore. I use clarity for early game so that I can stay and deal damage longer with my ADC. Not to mention also pull out the ult if they try and go in and push the ADC to mine while separating the enemy from his support, and if the jungler comes to try and gank I can push him away at well.

Exhaust > Clarity; Armor Reds, Health Yellows, Magic Resists Blues and Qrmor Quints
Better start with Stealth Ward and Zhonya's Hourglass and Athene's Unholy Grail are suboptimal because they aren't support'ish. Items like Mikael's Blessing and Talisman of Ascension/ Frost Queen's Claim are working better early on. Max Eye of the Storm at level 9 because it's your main ability to protect your AD Carry and enables him to deal more damage (about 45 attack damage when max skilled).