I do answer but I limit the negative replies to, "It has been a hard day" or, 'I'm a bit down." And of course I follow it up with asking the same of them. This gives them an opportunity to either delve further into your day if they feel the need to or an 'out' from an uncomfortable situation. Most I find acknowledge it and from their reaction I know if I can follow up in further detail. Remember that being uncomfortable is not a sign of not caring.