What type marble is it?-Hard or soft-makes a difference time wise when your grinding.
We charge a much higher price for grinding right to the wall than staying about 6 inches out from the wall.
Its the customers choice unless the edges are so lippy.
If the lippage is really bad and your going to damage your tooling you may consider grouting first then cutting it.
If you have a planetary you will do well-pull the moldings and grind the floor to the wall or just feather up as close as you can.
I agree with Carl on pricing about the same up here. Also we cut our edges by hand after we get as close as we can with larger equipment.
There are several techniques that you can use like setting back(mono-rotary machine) diamonds on the drive plate a few inches on the first cut-then moving the second cut up further.