In my experience, Shimano 8-speed shifters on a 7-speed cassette work indistinguishably from 7-speed shifters.
Originally Posted by
krobinson103
Done it with brifters and mtb bar shifters. Just tune out the top or bottom (I like the top to make sure it doesn't overshift) and the shifter doesn't care.
I wouldn't recommend this. The cable pull from the smallest cog to the next smallest is slightly greater than the rest to take up cable slack. It's harder to get good shifting if you don't start with the shifter in it's highest position. Just use limit screws to prevent shifting into the spokes.