The term "bare wood" means any wood that is devoid of any finish, either by wear or stripping or just never having had a finish. If this stair case is that old, I can't imagine that there will be no areas where the finish has worn through or has been chipped or gouged. Any area that gets wet with water will "take" the varnish or stain differntly and will appear darker than the rest of the wood. Some strippers are "water rinsable". this is fine as long as the entire wood surface is wetted, so that you have no uneven areas of color when your finished. I would stick to solvents for you initial cleaning as they aren't "wet" and will not swell the wood fibers. If your going to use a stripper to remove the old finish, then you'll be removing any discolored finish anyway. Whats left is the wood that will be slightly darker from the stripping process. Depending on how the previous owners cared for this staircase, you may have areas on the treads that won't clean up and even sanding may not remove it all. That's just part of the character of having something that's antique.