The best tool I've ever used for shaping my tips is a sanding block sponge with different grit walls. I couldn't find the exact one right away so here is something similar but instead of having different grits on different sides, each block has a set grit all around. For simple maintenance a fine grit will be plenty and resolves the biggest issue with shaping tools--that they are too rough and take too much material off which can really mess with softer layered tips.
I use layered Kamui tips so just gentle downward strokes with a fine sanding sponge at my desired angle while rotating the cue to get all sides evenly shaped is a great technique that's worked very well for me. It is easier to get it perfectly rounded than you might suspect.