The closest you want it to the bed is close enough to leave room for the shortest cutter and thinnest material. The nearest I can think of would be milling PCBs - so maybe 10mm. Realistically you can generally just put a spacer between if you can't get the spindle close enough, so I wouldn't worry too much.

If you put the spindle mounts closer together, you can slide the spindle up and down in the mounts and it becomes a moot point...