Well I installed my new NGK Iridium IX plugs the other day.. I torqued the plugs until they were tight, probably about 2/3 turn after finger tight with a spark plug wrench. The only difference I notice is that when the car starts now it revs up a bit higher, but comes down to normal RPMs. The car has 92000mi and this was the first spark plug change.

 

With experience you should get a feel for torque. Remember that spark plugs have a crush washer. The washer is designed for a safe installation: it does not have to be completely crushed in order to seal. Unlike bolts on, say, the brake rotor, the spark plug holds nothing together. No stress, no spinning.