Best practice is always have the space saver spare on the rear of the car. Prages nailed the exact reason: steering and control. The only time I might vary from that would be a truck with larger than stock tires, that is 2WD with LSD. In that scenario you're screwed either way. You can put it on the front and drive real slow or put it on the back and grenade your diff.
Flat spare: pretty common. All tires lose air naturally over time. Some more than others depending on the materials. You should always check the pressure in your spare no less than once a year.
The DOT says that a tire is beyond its safe operating age after 8 years from date of manufacturing as can be determined via the standardized nomenclature on the inner sidewall of all DOT approved tires. This is due to natural degradation of the rubber, and internal wear caused by the elements. Most older tires will have badly rusted steel belts due to years of condensation build up that allows moisture to penetrate the porous rubber.
The two most widely used types of rubber are Halobutyle and Chlorobutyle. The former is known for being less porous and therefore offers better air retention and slower degradation. The later being more porous, tends to degrade faster and lose air quicker.
Anything under the Goodyear family of brands (Dunlop, Kelly Springfield etc) uses the "H" type. Anything under the Michelin family of brands (BF Goodrich, Uniroyal etc) uses the "C" type, the same is also true for the Bridgestone/Firestone family, the Continental/General family and the Pirelli family (depending on individual tire model).
Pirelli also use a 5 piece centrifugal press to assemble their tires from 5 sections as opposed to the 2 piece/2 section Clamshell press that is the standard.
Most elite level super car tires (think Ferrari, Lamborghini, Bugatti, Z06 etc) such as certain Michelin Pilot Sports, Goodyear Eagle F1s etc use little or no actual rubber in their construction, but are instead the result of man-made compounds. These are most easily distinguished by the sound (or lack thereof) that they make during a lock up or burn out. Rubber tires "squeal", composite tires "scratch".
Is that tire nerdy enough for you guys?