I think graphite can be used when there is no other option
Watermark fluid, or more specifically lighter fluid because evaporation is slower, and strong light at an angle to the stamp while moist will reveal any grill.
Alternatively, careful examination in oblique light with 15x loupe should expose any grill points. Bank Note series grills are the most difficult to see. Some 1869 grills may also be difficult to pick out, but I haven't had much difficulty seeing 1868 grills. Unfortunately, few collectors have invested in high quality optics for examining stamps. If every collector owned and used a high quality 10x or 15x magnifier, returns of badly described stamp on eBay
, HipStamps or Delcampe would mushroom.
Adding graphite simply makes the back of the stamp dirty and creates a hazard for other stamps.