A safe design is a design that minimizes the risk of design errors. A number of techniques exist that make design safer
A design can be defined as safe if it is really what the designer had in mind and if it operates really as expected. As such, it is not related to design quality, the fact that the design meets its specification, but it is related to the fact that the designer did not make implementation errors.
The day to day design activity implies a huge number of actions that are not completely safe individually. The probability of making no error at all is then extremely low.
Examples of errors:
In case such an error occurs, a balanced structure can turn to an unbalanced one. For instance, if only one of the two transistors in a differential pair is modified, a structural offset can be added.
On the other hand, changing transistor sizes in more than one cell, (an amplifier and its bias) is error prone and boring.
One possible approach when many instances of a P-Cell appear with the same values, especially in more than one cell is to create a single component cell.
Such a cell contains only one P-Cell component. Its symbol can be given a look close to that of the P-Cell so that the schematics using it look like if using the P-Cell directly.
The benefit of this approach is that in case the one value has to be changed, changing it inside the single component cell applies it in all the instances.
More generally speaking, factoring common things is a safe design technique even though experience shows that only few designers use it.