I just joined so I haven't had jobs yet, but am I able to decline walks due to sudden unavailability, wanting to avoid a certain neighborhood, or dog aggression level? I don't want to be forced to walk in a neighborhood where I don't feel safe.
Yes, you can decline any job for any reason and should never sacrifice your safety. However, some declines (like not being available when the calendar shows you are) may affect where your profile appears on the site.
You can decline walks as you see fit but make sure you decline it before you book the walk. So if you think it won't be a good fit after the M&G, let the client know. If you book the walk and then cancel that will affect your ratings.