Hi Ara, same here. It bothers me when you loose a bit of the image, no matter how small amount. Thats why I like to mount my own slides, because you can adjust them in the frame to your liking and crop less desirable portion.
Just an anecdote, once I sent my slide for scanning and really wanted the whole image to show. So I took my plastic Gepe mount and with the knife carefully cut a small amount around the edges to enlarge it slightly, and the scan came out nicely