I had a student a few years ago who had taken lessons previously. She had been taught that you should use a standard Sharpie to draw the cut line on the glass - that's about 1/8" wide! THEN, she cut nearly 1/4" away from the line!! She said her previous teacher taught that method; she was prepared to grind 1/4" or more from every side. >sigh<
So, it's important to figure out the correct cutting pressure for each type of glass. With practice, you will be able to score where the cut actually belongs. Then, there are techniques to practice which let us break out the glass closer and closer to the score line. Finally, that grozing practice will get the glass pretty close.
I usually get mighty irritated when some "old pros" on the boards tell us that we should score & break perfectly every time and grinding should be seldom, if ever done. Often they are referring to pretty simple shapes, of course. Personally, I think practice, helpful suggestions from patient teachers and determination will get most folks well along their way to competence with confidence.
Glass on, Tom!!
Lazy habits translate the best!
My motto is to get it on the first run, grinding is seen as an option, but not the first. I would say only a tenth or less of the glass that goes into a panel I am building gets ground on the grinder. As your cutting skills inmprove you will find yourself using it less and less and the speed at which you produce pieces ready to foil will increase!
I also think that there is a big difference in cutting and shaping glass in what medium you are using to build the panel with. If you are foiling the edges need to be clean to accept the foil adhesive, if you are leading the edges can be chewed with grozing pliers and not very pretty at all if hidden under the lead face.
There have been many times when taking apart an old panel it looks like the first artist used his or her teeth to get the glass to shape!