In practice, these sensors are "nice-to-have's" but not essential. There are a number of ways to locate the new tool tip position. What I normally do is start from a home position with a set feature like a 1-2-3 block and z-axis handle jog my first tool to touch it off the top of the feature to locate the tool tip. Every time I have a tool change, I have grbl move back to home, and then carefully z-axis handle jog (don't touch x and y) up, replace the tool, and z-axis down to touch off the same feature. Just be careful not to manually handle jog any of the other axes during operation, since grbl will not know when you move them or by how much.