On passenger car and light truck engines with aluminum heads, valve seats
are usually factory installed with about .002? to .003? of interference
fit. Some say powder metal seats require a little more interference
fit than cast iron alloys, while cobalt alloy seats require a little
less because of their higher coefficient of thermal expansion.  

Keep in mind these numbers are for brand new heads with brand new
seats.  After tens of thousands of miles, seat counterbores can become
distorted and eroded, requiring an increase in interference to keep the
seat tight.  

The most common recommendation from valve seat suppliers for cast seats
being installed in aluminum heads is .003? to .005? of interference
fit.  If you are installing powder metal seats, use .005? to .007? of
interference.  If you are using beryllium-copper seats, go with .004?
to .0045? of interference fit.

Many valve seats have a radius or chamfer on the outside bottom edge to
make installation easier.  Seats with square cut corners are more
difficult to install and may damage the counterbore if they snag any
metal or become cocked while they are being driven in.

Chilling the valve seats in a freezer and preheating the head are often
recommended to make installation easier, especially if you are using a
lot of interference fit.  Using a lubricant also helps.  When heating
the head, don’t get carried away.  You only need about 160° to 180° F. 
If you get the head too hot, say 200° to 250° F, things can start to
move around and change the alignment between the valve guide and seat.