A couple of things to check - is the symbol editor set to High Precision (in the Symbol Property dialog) on save, this is needed for metric symbols on mm grids, it means you cannot use them in pre-iCDB versions of the product (2005 and earlier). If they are high precision then check the origin is also on a grid point, this might affect how the pins are placed.
If you move a pin in the symbol editor it will automatically snap to grid, you can't force off-grid items on grid if they're drawn that way, but you can snap symbols onto grid using the tool bar icon or from the Format menu.