It's of course up to how you feel and what you are comfortable with, but I have to say it's great that you ease them into it and what better way than at grandma and grandpas house.
My daughter was probably close to 1 year old when she stayed at my husbands familys house. It went really well, she slept in much later than what she does at home with us.. She loved the attention she got there, it's fun to stay with grandma and grandpa, they have different stuff to play with. There may be a time when you may HAVE to have leave him with someone else (not knowing what our futures hold) and it would be good to practice at a place where he can come home if he needs to..
It was hard, I worried and had my cellphone with me constantly and checked it often, but eventually I relaxed and enjoyed an evening with my husband out acting like it was pre-baby life (in between me checking the phone).