Depends on the vehicle. The truck is clearly in the "a little messy" category, with the more anal amongst us probably would be putting it at somewhere between "messy" and "a junkyard".
The PTCruiser is easily in the "clean" category, while the Miata ... a convertible ... is kept somewhere between "clean" and "spotless".
Because it's a soft-top, I keep nothing in it, and often don't lock the doors. Thieves are free to rummage around inside it via the open door, find nothing, and leave, versus cutting the top to get in after spying something through the window. In the summer, unless there is rain in the forecast (rare), I don't normally put the top up, even if parking on the street overnight.