Based on my own experiences, guards are very different in their attitudes, but the most common thing I've seen is "tickets from x (and y dependent on distance)".
Also, I've never seen any guard come through between Stockport and Manchester (as an example), mainly because I'd imagine they're unlikely to be able to do the full train in that time, and because Piccadilly has barriers/people on the gates the majority of the day.
Thing is, even if stations are gated, surely checking tickets is worthwhile, if for no other reason than to make sure people are on the correct train?