Although you can mark several cells for protection in one time, we don’t recommend protecting all cells: this would slow down execution of Excel. The goal of our formula protection is that if someone manages to remove the protection, protected cells will stop working and thus the workbook becomes useless. So basically, you should choose important cells to protect with XLS Padlock and optionally use the hidden attribute for all other remaining ones.
There are known tricks to deprotect hidden cells through password cracking, but this requires VBA. If you block access to VBA editor with XLS Padlock, this should be OK.
Note that it is possible to disable the formula bar in XLS Padlock but this doesn't really secure formulas.