If you are going to record with M/S in a room with an ensemble... you are going to get the ensemble.
So, you can place the mics so as to get the whole ensemble...
Or, you can place the mics close to a soloist and use the M/S mixing in post to dial out most of the sides and still have some.
In practice, most folks use M/S recording on an entire ensemble and then use the post mixing to dial in just the "right" amount of room. This is really useful when the room isn't very good sounding or empathetic to the musical content. It is also intoxicating when the room sounds great as you can dial in more of the room and use that as reverb.
When using M/S on a soloist I imagine most folks do it without other noises in the room and focus on the featured musician. Then you can use the M/S blend to get a nice rich sound with some stereo sparkle.
The idea of spot micing someone in a ensemble with an M/S Array is probably an exception rather than the norm. I've seen it done, and done it, with drum over heads but that is a specific situation where the bleed from other stage instruments is expected.
best regards,
mike