I've found through experience that the thinnest you should go in the letters line thickness for stainless steel printing is about 0.4mm to have good detailed text. This is a good minimum thickness for both engraved and raised letters in my experience. I've pushed a few of my models thinner to about 0.35mm and had the text start to fade and bleed together at points. At 0.4mm you will have to spend some time cleaning the polishing medium out of the text though and may have some pieces that are extremely difficult to remove as they get wedged in there so tight. These days I usually try to stick to 0.5mm thickness for my engraved lines now as it is much easier to clean out the polishing media at that size. If you are having the items plated in gold I would not go below 0.5mm in thickness, as the Techs at shapeways have to clean the polishing media out of the model before it is plated and may not be able to get the difficult to remove pieced out of the engraved lines leaving you with unplated areas that will be exposed later if the polishing media falls out or is removed.
For silver prints check shapeways' materials page for their minimum detail level.