I use a razor blade for this kind of thing - but not the shaving kind. You can buy razor blades that have a normal very sharp edge on one side, and a shielded, or protected side on the other. They are made for doing hand cuts, or even shaving of over painted areas on windows, etc. They are also made to fit into handles for the paint shaving purpose.
The url below will take you to someone who sells them, but you can find others who sell them, they are cheap. I found it by Googling "razor blades graphic arts". They are great to have around for finessing bevels, as Bill suggested, and other trimming. A box will last a long time.