Whenever someone buys wedding gifts at the store, they always ask me to put in a quarter.
I would worry more about what the bride and groom think than we do. I've been asked more than once for the gift of knives, but some people have no enthusiasm . Often times, they wanted sets I wouldn't likely recommend. Some barely set foot in a kitchen. You probably know who you're for.
2nd what everyone has said about including a coin so the bride and groom can "buy" the knife. I think it's a cute tradition.
For simplicity, though, one of my friends just gave me cash with the instruction to buy a good knife That worked too!