I've kept a pocket knife in a leather belt case for years. More than one, though I never wear them anymore. No issues there either. TI and Hart, to name two, furnish a leather case with their razors. I suppose if it was going to be an issue they wouldn't do that. Just the same I never store my razors in leather, though as Bill said, "People have expressed concerns about acids and tannins from the tanning process." That, or not being careful to avoid exposing the razor/case to moist environments, might be an issue. Fortunately the companies that make leather cases for razors seem to use the leather tanned with the process that won't leach corrosives onto the blade.