Operating systems generally decide to route packets based on a routing table. On a Linux (possibly even Mac?) box, you can type:
$ ip -f inet route
default via 192.168.1.1 dev wlan0 proto static
192.168.0.0/16 dev wlan0 proto kernel scope link src 192.168.1.107
192.168.122.0/24 dev virbr0 proto kernel scope link src 192.168.122.1
To see the routing table. This tells you that packets addressed to
192.168.0.0/16 go down wlan0,
192.168.122.0/24 goes down
virbr0 and everything else (the default route, sometimes called
0.0.0.0) also gets sent via
wlan0. It is assumed by the kernel that whatever is plugged into the other end of
wlan0 can answer, of course.
What this means is that depending on your routing table, the OS will do different things. If your default route is via your ethernet card, the OS will only use the wireless link when it is up for resources on its network.
There are two considerations for leaving the wireless card enabled, however. These are:
- If the OS has been configured to automatically connect to an access point, you may find that the card being enabled allows it to automatically connect, which may not be your intention. This may also re-route traffic depending on your routes, which also might not be your intention.
- If the wireless card is enabled it is likely the wireless card will be used to periodically probe for access points the OS knows about. This information can be captured by anyone within range with a wireless card in promiscuous mode and therefore might prove useful in an assessment of your network.
If I'm honest, I think the risks of leaving your network card on are quite low, provided you have correctly configured your wireless network (you're not using WEP, are you?) switching to your wireless is unlikely to pose a horrendous risk to your security. If it does, then I'd ditch the wireless network altogether.