NO NOT obsess over the piano.
If you are submitting songs to Nashville publishers, you should understand that in Nashville, more than any place else on the planet.... it is about the song.
A nice, clean, well produced "demo" will work just fine..... from a full band production to a simple well recorded voice/guitar or voice/piano demo.
No one is going to listen in Nashville and say, wow that is a really good song but that piano sample is a Bosendorf.... I wish they had used a Steinway, then I would have signed the song..... nope that is not going to happen.
You want the song to be the best it can be. Because you don't want them saying wow, that is a really nice sounding piano but the song sucks...... next....
Wow them with the song.
Personally, I don't even worry about the piano parts... I use the cakewalk steinburg collection.... I paid $20 for the samples. So far in reviews of my music from Nashville's hit songwriters, not one has commented on the piano (good or bad) but they have made comments of the song itself.... because in Nashville............ It's all about the song.