pcb - Is case grounding compulsory in typical 32.768kHz crystal for Real Time Clock?

Pictures shown below is a typical 32.768kHz crystal, commonly use in Real Time Clock circuit (e.g: DS1307 & DS1337)

Referring to an earlier question posted here, it is a good practice that ground planes should be beneath the crystal. But is it compulsory that we should ground the crystal's body/case too (like what they did in these pictures)? And if yes, what happens if we didn't grounded the case?