Different processes will require different diamond disks. On average, if you are working on pieces that are anywhere from 1 to 3 inches in diameter on the area to be ground, then you can usually start off with a 60 or 80 grit for the rough grind (60 grit giving you faster removal, but with more edge chipping; 80 grit giving slower removal and less life but with far less edge chipping).
You can then move to a 270 grit for the fine grind. Then to a 325 grit smoothing pad for the pre-polish, and either a Synthetic Felt, or LP material for the final polish with cerium. A quick and efficient four step process from rough grind to final polish.
For surfaces over 3 inches in diameter, we will generally recommend inserting a 100 or 140 grit between the rough and fine grind disks. This keeps everything running equally and will prevent you from replacing your 270 grit disks prematurely and often.
You may also find with larger work that a 220 grit smoothing pad after the 270 grit electroplated disks and before the 325 grit smoothing pad will also make your grinding proceed faster.
If you are grinding very small pieces or coming from a good saw cut, you can actually start at a much finer grit like a 100 or 140 grit. If you are only grinding edges, you may even want to start with a 270 grit (as long as you don't have to remove too much material from the edge).
Extremely large surface areas or areas that require a lot of glass removal may benefit from a 45 grit disk, although these are rare. It is generally better to try and remove large portions of glass with a saw before trying to grind away a large amount of glass. A 45 grit diamond will remove glass very quickly and create a tremendous amount of edge damage while doing it. Oftentimes it is better to start extremely rough grinds with loose grit on a Rociprolap.
We also carry 325 and 500 grit electroplated disks and we've had many customers say how much they like the surfaces that these disk give to them. We do caution most glassworkers though that 325 grit and 500 grit electroplated diamonds are very very fine and any small contaminant between the disk and the glass can leave a pretty nasty gouge. These disks are generally much better suited for very small fine work or lapidary work.
We also have 3M's electrostatic diamond disks available for use on smaller pieces like jewelry work or murrini and millefore. Available in 8 inch and 12 inch diameter, they perform amazingly well on small surface areas and give a clean precise grind that is easy to control and easy to polish from. They will not perform well on larger surface areas, so don't expect them to last if you're working big.