I've forgotten what a difference that makes. I just added more water after our pair of 80 degree days, and many plants that were "sitting on the fence" decided to come out and dance. Mixed metaphors are always best. I have done this many times and as a result, many plants took a graft when they weren't going to. I first noticed this on ones I lost. I guess if I were a pro, 95%, extremely precise grafter, this might not make so much of a difference. Even on cloudy days it makes a big difference. If it doesn't get enough water, I think the plant thinks, wait, drought coming, better throw off any distractions, which may be your scion. Watering it ensures that your plant can "afford" the water to the scion.
John S
PDX OR