RE: When to replace motor mounts?
Another way to check how the mounts are doing is to check the shaft alignment (assuming it hasn't been readjusted in the recent past).* If you disconnect the transmission from the prop shaft and they're still in good alignment, the motor mounts haven't sagged and are still happy.* If not, before realigning, replace the mounts.
However... you will need to do the whole engine/shaft alignment process two times if you do so (once at installation and again after a few hours when the mounts settle a bit).