Hi coach white and Ziggy and welcome to the grey forum family.
To answer your question about shoulder sitting being bad. IMO no it`s not bad, but there has to be rules.
First how old is Ziggy and is she/ he a rehome or are you his /her first home.
Step up is the most important thing you can teach your grey IMO.
When she lands on your shoulder will she step up onto your hand ?
Does Ziggy bite, If he /she does than no shoulder sitting, you will have to block your shoulder with your hand so he/she can`t land there.
Tell us more about Ziggy and we love pictures.
P.S.
I have a congo grey, a BF amazon and two cockatiels and to them my shoulder is the best place in the world. But they do know the rules and will step up when it`s time to come down.