No you won`t hurt anything at all. The transformer thinks 220 is coming in and steps 220 to 12. If 120 is the actual input, it will end up 6 volts and you will get about 10 years from the bulb.
I think there are 4 setting, 220, 240, 110. and 120. If you are happy with the dimmer light from 220 use it. If you want to go to larger prints, select 120 for a brighter light.
Test first if you use VC paper as the color temp gets redish at low voltage. This could change the contrast as the paper is color sensitive in blue/green. Change the ratio, contrast changes.
I used a voltqge stabilizer and set it to 120. I don`t remember burning out a bulb. Get a stabilizer if you do color.