Some grafting and budding methods won't initially require heading of the rootstock. T-budding, chip budding and bark flap grafts leave the entire rootstock intact until the bud/graft is forced. Cleft and similar grafts do require heading of the rootstock, however the higher the graft is done on the rootstock stem, the more rootstock will remain available for a retry in the event of a failed graft.
In the event of a failed graft on a low rootstock position, suckers should develop that can be thinned to a single stem and regrafted, typically after losing a season waiting for the new stem to mature to the point of being suitable for grafting.