I use Lucas Slick Mist on the whole bike including the Shield. Two and a half years hasn't hurt it yet. Its safe for plastic. Rain runs off and the bugs come off easier.
Another fan of Lucas Slick Mist here. It's the best quick spray I have ever used. Running up and down the Alcan and Cassiar highways it keeps my bike looking good and the nasty north country bugs come off easily.
2010 GL1800 Mine
2009 KLR650 Mine
2009 XR650L Hers
1996 GL1500 Hers