I'd be a bit uncomfortable with a registry to be honest, I prefer the idea of an informal list.

Although you could have a registry, just not tell people about it, if friends contact your Mum or MIL wanting gift ideas, then they could be directed to the registry. If people have their own ideas about what to get, then they can do their own thing without feeling obliged to buy from the registry.

For myself I couldn't send out registry cards with a shower invite, but then you don't want to get loads of stuff that you won't use either.