I was an instructor and pass holder at Sierra for several years and assuming there's snow, you will find good trees all over, and some fairly steep runs off of Grandview Express as well as over in Avalanche Bowl. The West Bowl also has great tree runs.
The backside trees are fun as well (but not steep, and it probably won't be open by March the way things are going this year).
I've only been to Kirkwood a handfull of times, but if you want steep you can get it there assuming there's snow. I did jump off a 6-8' cornice there once. Really fun on a pow day, but alot of the lifts suck and you have to be clairvoyant (or really local) to figure out when they open up after a dump.
Not sure about tree runs, but there's a fun gully or two there that I can remember.
Never been to Bear Valley but I've always wanted to go.