I use a chain stretch tool. Even when I replace the chain at equal to or less than .75 my cassette is knackered usally on the the 3/4 smallest rear cogs, my most used gears. However the .75 switch seems to save my front rings. So far anyway but still testing.
if you go for a new chain only- keep the old chain! If the new chain skips I would be tempted to repair old chain and run it on old cassette until you get obvious shifting problems and or hit .75 wear than change both at same time.
Just my opinion though, I have looked into this a fair bit (from a novice viewpoint) and it seems to just come down to how hard you personally wear this stuff out.