I don't think it's a bad idea, although to confess, we haven't done it and won't do it until we have the kids' college educations paid for. But it's a good way to help ensure that you get the kind of burial you want to have.
My FIL in died a year ago. He had always said that he wanted to be put in a box and buried with no hoopla, no ceremony (he was not religious), and in a frugal way. He was a pretty frugal guy, but traditional enough that he didn't want to be cremated or donate his body.
Instead, he was embalmed and tarted up, put on display at a wake, had a fancy coffin, a church service, a headstone, etc. Dh pays MIL's bills for her and the whole thing was many, many thousands of dollars. I'm sure he was rolling in the grave.
Why? My BIL is a bully, he's religious and has bought into the traditions of his wife's family, and he put all sorts of pressure on my MIL at a time when she was very vulnerable. Because of the sadness of the situation and MIL's stress, nobody stood up to him.
I've heard too many people say that "funerals are for the living" as a justification for disrespecting the wishes of the deceased.