Always worth asking for all tickets, I even ask to see railcards again - I've found the odd out-of-date or missing one, depending on whom I've taken over from. Also, it's not uncommon to have the "You've checked it" - "No, it wasn't me" - "Oh, I seem to have lost it" - "Oh dear, you'll have to buy another one then" conversation.
Very true. I have taken over from colleagues who have said "All ok, all tickets done, everything fine", then gone through done a full check and found out of date or non existent railcards, folk with advance tickets on the wrong train, folk with no tickets at all, adults with children tickets etc etc. And often the invalid ticket has been stamped by the previous person who hasn't spotted something.
And it's amazing when I ask to see railcards of people who have had their tickets checked already how many times I hear "well no-one else has ever asked to see it". This is normally when I have discovered they haven't got one in the first place.
I once had a chap on a train who was doing a very long journey with a very cheap advance ticket and had been on at least 3 trains before getting on mine and had 3 different stamps on the ticket. I asked to see his railcard it soon became obvious that he didn't have one. He couldn't even describe what one looked like. It came to light that his son had purchased his ticket on the internet for him and had assumed that his bus pass entitled him to the senior fare. The problem then came as I pointed out I would need to charge him £xxx as his ticket was invalid, but he then said "well the ticket has been seen by everyone else and no one else has ever said anything on the other trains". Cue long rant about how I was being a jobsworth, if no-one else is bothered why are you, you can see I'm a pensioner here is my bus pass, I'm going to complain, I want your name, No one else has asked to check the railcard blah blah blah.
If everyone did it correctly and thoroughly all the time it would save a lot of hassle for those that do!