how dow I know "whether the serial makes sense"?
Figure out what you are sending and determine if serial makes sense. That's not really something you can google and get an answer.
If it really is a small amount of data or it isn't time sensitive, then maybe serial does make sense. However you have to weigh that against your issue of having people install something. Look around the forums, there is a user who has a modified IDE that sets up the serial drivers (which aren't really drivers, it is just an INF file) automatically. Maybe you can do what he is doing.
Can you please give me a link to an example of a PC program, preferably in C++, which talks to Arduino over USB without a driver?
No, I can't. Why would anyone do that? There's a good chance most operating systems wouldn't allow it or require some pretty interesting hooks to allow it. You would do what every other hardware manufacturer does: write a driver for your USB device and communicate with the driver. There are many "protocols"/profiles/stacks already built into operating systems. You can probably use one of those. (e.g. Serial)
And I'm just curious, why Micro would be different?
Because it doesn't have a FTDI chip?