I use a diamond plate for raising slurry on Naniwa stones - I think it helps in seeing what's happening but more importantly it also seems to prevent build up of swarf on the hone. I can't see any problem with using a slurry that's the same grit level as the stone currently in use - that's why I generally use the diamond plate although just recently I also used the 'nagura' that Naniwa sell, for slurry as well as cleaning the hones.