Originally Posted by cbfight

Shimano tried for a while to spec triple shifters for both triple and double applications, but discovered that too many people were adjusting the triple shifters incorrectly for double applications, thereby making them more prone to failure. The specific issue was that people were using the 1stand 2nd clicks for shifting between the rings in a double instead of the 2nd and 3rd clicks. Problem is, by using the 1st and 2nd clicks, you run the risk of mangling the shift mechanism by attempting to click into 3rd, which would obviously be prohibited on a well adjusted FD by the high limit. Plastic shift mechanism vs. metal limit screw and you have a broken shifter.\

Setup the shifter to use the 2nd and 3rd stops and you'll be fine since that way the shifter will naturally bottom out when it should. The first click would just take up extra slack. It's a perfectly safe thing to do provided you do it right.