Shading can be bad for a system. There are different kinds of shading, which are determined by the level of focus of the shade. For example, "soft shade" is from far-away objects, whereas "hard shade" comes from objects that are closer to the solar panels.
If you're trying to determine just how much shading is going to occur on your roof, then you can get into using a device like a Solar Pathfinder.
Here are what obstructions appear like, when you are using a Pathfinder:
If you don't have a pathfinder, it would be not as accurate but you can guess your shading. Many people seriously considering systems will effectively analyze their roof for shading just by observing it year-round, and you can't beat that. If you check right around the holiday season, to see where all the shadows fall on your roof, then you will probably have an idea of the worst of your shading issues.
Or you can get a pathfinder! It's entirely up to you.