Let's say you want to cut around a jar which is very curved, a traditional glass cutter wouldn't work well, so what would you use?
Specifically for cutting glass jars and bottles
A very common method is to use heat shock to break the glass; normally by putting a ring of some form of absorbent string or twine around it, soaking it in acetone or some other fuel, lighting it and then plunging the bottle into very cold water. Some tutorials recommend that you use a glass cutter just to score the surface lightly to increase the stress, so it seems that using a glass cutter might also be possible (if difficult).
For cutting arbitrary curved glass surfaces
Honestly, a traditional glass cutter or dremel is probably still your best bet. If you're struggling to hold it steady on the curved surface, consider covering it with masking tape to give the piece a more friction - this should make it easier to cut without slipping.