I notice that this has changed in recent ages. Before "Hostile" tags kept some people out, but not everybody. Now, people rarely hit into hostiles. Perhaps its because I went from a "ghetto" to a "top kingdom" and people near the top are better sports, who knows...
If it's an irresistible grab, like you'll gain a HUGE number of books, then I say it's worth the "bad form" and hit them. If it's just for land it's generally not worth it. There are plenty of other targets for land and resources that aren't in hostile