Dave

It really depends on how often your shower is used but in most cases a normal electric domestic shower will not need to be tanked but if it is a power shower then i would IMO...if a shower is used a lot then the grout does not dry out and then water ingression happens.....there are a few ways around this.....

The moisture resistant plasterboard is simply that ..resistant, not waterproof so if the shower is heavily used then i would use extra protection for the substrate...be it a tanking solution or membrane.....the call is yours after all......