Usually I find tabs I want are next to each other, or easy to drag together. If there are a log of tabs in the window that I don't want then I use the Tab Split Extension[1] to put them in another window, or I just edit the bookmarks once I've created them.
[1] https://chrome.google.com/webstore/detail/tab-split/imjbfepo...
Better yet, a "Bookmark similar subject" plugin would be awesome - Leverage Google's search to automatically figure out tabs that are about the same thing and bookmark them by subject. Ignoring pinned tabs would be a bonus.