Softer knee I use when I need to make a compressor when I need the transition from compressing to not compressing to be soft or the opposite when it to be sharp.
things like drums depend on what im working with.
vocals I use a ratio or 1.5.1 with pretty low thresh and mild knee (depending on what im working with)
things like synths and instruments, I usually dont compress often, but I low ratio of 1.x.x to 2.0 and a mild or soft knee depending on what im working with just to make it sound warm
on my master track, I usually don't compress and just EQ unless I think the track needs compression to add some warm, power or something. (or if I want to add something/fix something to the track, a multiband. usually when I want a band to dominate the others which im starting to do more often.) then I limit depending on what song/genre im working with.
I really only compress when I feel I need it.