All modern bikes are designed to handle a significant amount of rain, both moving and standing still form. But to maximize the life of the bike and/or parts....it's best to shield it from extended water exposure. The issue is, water will creep and settle in all sorts of areas, which can lead to electrical issues or small mechanical problems such as key-tumblers may get sticky or rough due to rust.
For me personally, the only time I cover my bike is when I park at the airport. This is mainly to shield the bike from jet-fuel particles that float around and can leave a residue on the plastics and seat when I get back home from my trip. Outside of that, I rarely use a cover, even a multiday trip with some rain exposure.