Leveling Shenhe from 80/90 to 90/90 would only increase her Base ATK by 21.36, which translates to an increase of around 50-70 Total ATK after considering ATK% artifact stats, which generally is a ~0.8% team DPS increase. While extra ATK is nice to have, it’s not necessary, so generally only consider ascending and leveling Shenhe to 90/90 after you level her appropriate Talents, upgrade her artifacts, and invest properly into other characters you use.