A +1 from me for a switching regulator on a module, they are so cheap it's really not worth the effort doing anything else.

As mentioned 12-5V is a fairly large step down for a linear regulator. Even 100mA will be 700mW of heat which is quite a large amount.

A car USB charger is a good idea too for up to 2A or so which would be well out of realistic range for a 7805.

Presuming this is for the BT empeg project, how much current is the BT module rated at?