Some people have a slightly different body chemistry / electrical resistance, and can build up higher levels of static than average, and the clothes you wear (especially the soles of your shoes/boots) can make a massive difference. Thing is with static, you can cause microscopic damage which has no obvious effect, but can reduce the lifespan of the equipment, or reduce its potential for overclock.
Wristbands are cheap, and reduce the chance of a low level static shock causing small amounts of damage.
BTW, Touching a radiator is a very bad choice, many radiators these days have plastic piping under the floors, and is only copper piped where the runs are visible. If the rad is plastic piped, and doesnt have an earth strap it wont help at all!. Also the paint on most white radiators is a good enough insulator that you wont lose the static charge, so if your gonna take the radiator method and you know your house pipes are copper, and earthed, then touch the copper pipes, not the radiator. (and be carefull in winter, those pipes can be very hot.. that should cover me against injury claims!

).
The best way to be sure of a clean earth is from the earth pin on the mains, so having the PSU installed, and plugged into the mains (with the main switch on the wall OFF) you have a good earth, and that earth will be available anywhere on the metal frame of the PC, and its safe enough. (To be honest your safe with the mains on too, unless you are actually opening up the PSU.