I've got three different mug moulds:
A commercial one, a home made one, both with handles, and a home made drop-mould without a handle.
When it comes to fettling the rim, the two with handles give me problems. No matter how I trim the slip from the pouring spare, I'm left with an ugly edge.
With the drop mould I either trim level with the top of the mould, or when it's firmed up a bit, I use a scalpel in a height-adjustable stand to score a new edge and keep going until it's cut all the way through/round, then round off the flat edge with a sponge.
With the other two I spend much more time getting the edge level, same thickness all the way round and nicely rounded off.
If anyone has any tips I'd love to hear from you.