It all depends on the audience. Those would play RPG games using miniatures would like to be able to print the map out, so 300 DPI is optimal for making a printable map for use as a battle mat. Then, there are those such as myself who exclusively use a VTT(MapTool in my case) and a 300 DPI map would be totally useless for across the internet games due to the file size. For those people, 50 to 100 px is optimal, typically without the grid as most VTT's put their own grid down to control movement of the tokens and make sure they can line up.