I am getting ready to lay 3/8 underlayment over aspenite and have some dips in the floor. Should I shim the 3/8 plywood and work to get that flat or use singles/felt to get the 3/4 hardwood flat. I am thinking that if I shim the plywood then I run the risk of the plywood sagging between the shims. Thanks for your advice.

I found the answer in my ealier post, you level when putting down underlayment.