As you say, how long is a piece of string.
My view is that if it's something that you could have picked up from the viewings or would expect from a property of that age/state of repair then a post-offer reduction is a bit cheeky. If it it could only be picked up by surveyor/expert and is unexpected then fair enough, try for a reduction.
People can be strange though, they've agreed a fee and aren't budging or maybe they have a bottom line figure for their next purchase and won't go lower. It's tough to make a call, you can probably push for lower but you may not get it.