Depends on the country. The usual requirement is for copies of the General Declaration to be submitted to Immigration, Customs and Health authorities. Where these functions are combined or split (eg. Health/Agriculture), additional copies may be required.

From personal experience, some places (eg. France) really don't even care about the General Declaration. If you look like you're wearing a uniform, they wave you through. Others (eg. Angola) are a lot stricter and they scrutinise the lists very closely, especially the Health declarations. Most countries just do random checks for customs purposes though, similar to what they do on the passenger side.