I used the term "safest" to describe the method of least risk. Any method of attaching wire to tape poses a safety risk if it is done incorrectly. Anytime you puncture the metal strip inside the tape you are degrading the effectiveness of the product.
The problem over time will be durability. If the tape is subjected to constant movement, such as sliding tubs over it, the screw's hole (or the clips prongs) will eventually become stretched out and elongated. This causes a break in contact, loss of heat and sparking is also a nice little possibility.
Solder permanently affixes the wire to the tape. In all the years I have been soldering the connections I've never had one break, come loose or otherwise malfunction. I've tried the clips and I just don't find them to be as effective, especially for as much as they cost.
Again, these are just my opinions and are based on past experiences.