It never hurts to look at rings together at some point to get a sense of what she likes, especially in the cut of the diamond (round, princess cut, asscher, etc.), the materials for the ring (yellow gold, white gold, platinum, etc.), and other diamonds.
For me, I got a sense of the diamond cut and material (she liked Platinum, surprise). Then I chose everything else. However, I let her custom design a bespoke wedding ring (not the engagement ring). So I picked one, she picked one.
I think chicks love it when you make some of the choices, as it's a gift.