Question: How do I disable the auto-updating of the Top Sites in Safari? I don't want to turn the feature off altogether, I just want to prevent auto-update. I know this may seem like an attempt to defeat the purpose of Top Sites in the first place, but I find that if it auto-updates any of the Top Sites that are forum-based, all of the unread topics are marked as read. So, an even better solution would be the ability to exclude the auto-update of certain URLs/domains, etc. Does that functionality exist?

Answer: 
To disable TopSites, quit Safari, open Terminal and type or paste:

`defaults write com.apple.Safari DebugSafari4IncludeTopSites false`

Press Return key. Relaunch Safari.
If you use Safari's Private Browsing mode, Top Sites will not update at all. However, this solution might not completely address your original question - I haven't found any solution that could turn-off auto-updates of Top Sites, either completely or selectively for certain URLs/domains.