When to acknowledge ATC

ATC says "radar contact, 20 miles south of XYZ, proceed on course". Do
you acknowledge this transmission? How about when ATC says "altimeter
setting 2992" on your first contact after a handoff? Does this require
acknowledgement?

In the past, I've acknowledged such things if the controller was not
busy. But I've heard all kinds. Some people read back the altimeter
setting. I've even heard people reading back the "radar contact"
message. I feel that this is a waste of bandwidth. However, I don't
know what ATC prefers. Do they want read back for everything, or should
we shut up as much as possible?