It depends on where I'm going, and since I'm not old enough to carry concealed, it goes under the seat in the car (unloaded.) If I'm just running out to the store I'll take what's in it (one 12rnd mag+1 in the chamber, it's an XD40 Service.) If I'm going to a bad part of town I'll slip my other extra mags (two) in the XD Gear case it came with and put that on my belt (while keeping the XD holstered under the seat if I need to slip it onto the belt.) With the way things have been going around Blacksburg my Dad or Grandfather (the people who I'm normally with) will also be carrying, Dad the 9mm Sigma, or my Grandfather the P22. Neither of them ever carry extra mags.
If I have the Snubby .38 I'll slip it under the seat along with a box of ammo because normally when I have it I'm spending the night at Mom's and want more than five rounds or I'm going to the range, etc.