I haven't used the scheme you came up with before, but I really like it. The only downside I can see is that without the context help open, you lose visibility. Even then, you'd have to mouse over each VI to check the terminal requirements. If you're just eye-balling a block diagram, especially a big one, that can be a bit unwieldy.
In the past, I've followed NI's example of Invoke/Property Nodes and added a little red "?!" to the icon if my subVI is going to ignore error.
There's no option to ignore incoming error, but obviously you can do whatever you want with your own subVI.