Bending forward (flexing the spine) to touch your toes is NOT a good stretch. Yes, it stretches the back muscles, but it puts your spine in a compromising position, which carries high risk for low back injury. This is especially true if you are standing and bend forward or picking up an object in a forward bent position. The heavier the object you are lifting in the flexed position, the higher your risk for back injury. A back injury (such as a disc herniation or a compression fracture) can result from 1 traumatic lift or simply from habitually bending forward.
You already do a lot of forward bending in your daily life (unless you practice perfect body mechanics, which I recommend). Each time you bend forward, you increase the pressure within the discs (between the vertebrae). You also lose the balance and stability of the spine, which is strongest when it is straight (AKA neutral spine posture). When the spine is neutral, the vertebrae are stacked up and the tissues surrounding the vertebrae are in balance (front-back and right-left). This is the position you should be in when doing any heavy lifting. Have you noticed how professional weight lifters perform a 400+ pound squat?
It is especially important not to do forward bending activities early in the morning, for the first few hours after you wake up. The next time you are tempted to bend forward, try bending backward instead. Also, if you usually stretch your hamstrings by touching your toes, there are many alternative hamstrings stretches that are better for your back.
Another position that puts your spine in the flexed position is sitting with slouched posture. When you combine excessive slumped sitting with excessive forward bending, you are asking for low back pain. But the good news is that it is not too late to change these habits!