Most guards doing on-train revenue checks are reasonable people and make good use of discretion, so your approach is probably not wrong. For instance, I had no massive issue with showing the Inverness conductor on the 10.56 Kyle train the ticket for the 08.55 despite the lack of endorsement, as those guys are generally a nice bunch and are probably aware of issues with the sleeper anyway (and the near impossibility of prosecution for a ticketing irregularity in Scotland means that you can be more relaxed about such things).
It’s generally always been from RPIs on Intercity routes out of London that I’ve had or known grief from in the past, particularly EMT. I did also witness a PF being issued on a Norwich to London over an invalid advance (I think a connection from one of the East Anglian branches was late so they missed their booked train). Very surly RPI insisted that the ticket had to be endorsed otherwise it was invalid and a PF would be issued... young lass was quite distressed and the RPI did absolutely nothing to help except say ‘you should know that it’s against the law to travel without a valid ticket’ repeatedly in a condescending manner, take her details, issue a PF and collect the £20. The girl tried her best to explain the situation but the RPI was having none of it (not that he actually cared to listen). Then he told her that the full anytime single from Norwich to London was due... young lass had an absolute fit and other passengers intervened telling him he was being ridiculous... the RPI then threatened to call BTP (obviously a bluff)... eventually the RPI said he’d be ’lenient’ and let her get off at Stowmarket and buy a discounted off peak from the booking office... which the poor lass did (delaying her journey needlessly by 30 minutes, or perhaps more).
Having witnessed the behaviour of that RPI (whose behaviour was actually out of order, and he actually caused someone to delay their journey further) I decided if travelling on that route in that situation to save the hassle and buy a new ticket. What must managers think of how some staff (particularly RPIs, as guards generally tend to be more relaxed) behave towards members of the travelling public who are clearly not criminals and have actually been inconvenienced by the rail industry? It’s definitely a poor advert for the industry. Particularly when the passenger is not even vaguely in the wrong...