Redrobes, my tiles are 100 ppi
From all my visits to RPTools and discussions with RPMiller, I"ve come to the conclusion that many will be requiring only 50 ppi, but some people (RPMiller) wants 200 ppi, as MapTools allow you to zoom in and only 200 ppi really shows closeup detail well.
However, a general consideration is how long it takes to load a new map. I've created some early VT maps at 200 ppi, PNG and they were 10 - 12 MB. I was told that the expected file size should only be 1 MB for easiest downloading for most VT users. This just isn't possible for 200 ppi.
Since some users desire a higher res file, but don't want to pay the download hit, I've compromised to 100 ppi. My Dungeon Tiles are around 1.6 MB that's close enough to 1 MB for my taste.