Indeed they are safe to use however I wonder why you need to use them? I use them only in tanks where I am providing temporory cover - e.g. in a breeding tank to hold the plant in place. no gravel.
When I plant in my tank there are always enough roots on the bit of plant stem to hold it in place. If planting bare stems, they tend to root themselves in no time at all.
Problem with using these weights is that they can strangulate the plant stems/roots as they try to grow. I find that I end up with plants dying and rotting away leaving the lead strip in the gravel. I then have to run my fingers through the gravel to find the lead.