Currently, anything that shows up in “Incoming Shares” will show up on each computer you have Insync running on. However, upon taking action on an incoming share, such as clicking “Sync to this machine” or “Don’t sync”, the share is only dismissed on the current computer.
I know at least in my case, when I make a decision on a shared item, I either want it on all my devices, or none of them; if I make an action on one computer, that action should apply to everything.
There’s probably people out there that do appreciate the ability to selectively choose which shared items get synced to which computer in that dialogue, so if this feature were to be built, it might be a good idea to make it a preference.