I believe the 32-bit id you select needs to be unique from all the ids your OS knows about or else it will install the wrong driver and may crash / encounter other issues. Of course the user could manually uninstall that driver and install a driver you provided.

But randomly selecting an id doesn't make much sense. For example here are all the ids Linux knows about: http://www.linux-usb.org/usb.ids. Obviously don't select Logitech, Microsoft, Canon, Samsung, etc. manufacture id. Either select an unused manufacture and then randomly select the device id. Or select a manufacturer id that is no longer used like US Robotics (I doubt they are making any new modems) and then randomly select a device id.

As soon as make your device, connect it to a Windows machine which will try and find a driver -- if it doesn't, probably no conflict. If there is a conflict, it's with a product that distributes it's driver manually - so you don't have to worry. Or a product that doesn't exist yet, which could be an issue.