In general, I have to say that I'm spectacularly unfussed by people putting bags on seats. The main thing is that they move them with good grace when asked (actually, it's quite fun to mock them when they don't, but you pick these things carefully...) Why are people so afraid to ask someone to do this, or to let them into a window seat?
What I find far more annoying is people putting bags in the aisle, and in other people's foot/leg space. The commuter trains I go on are terrible for this. The weekend is worse, with suitcases occupying entire bays of 4 (and the GN 365s do actually have quite decent luggage space...)