Some people like to limit it a little higher than the refresh rate, I guess so when something explodes or something they don't notice the FPS drop. To me I don't really notice a difference between 30 and 60 unless I'm really looking for it. But then I'm just seeing my hard drive micro stutter
fps drop will be the same in both occasions. theres no difference from 60fps>40 fps and 90fps>40 fps on a monitor refresh rate of 60hz because only 60 frames can be shown, which means in total, both will only lose 20 frames. the only difference between the two is that the one of the higher fps has more samples to pick one, and skips a few since it can only display 60 of them, wheras the 60 fps will display all of the frames it generates.