1. There is no need to mention these words at the place of your signature though mentioning such things is advisable.
2. However at the very beginning the name of the principal on whose behalf you are signing is required to be mentioned and I am sure in all those documents it has been duly mentioned.
3. if that is so then it is perfectly alright and failure to mention it at the bottom of your signature would not cause any problem.
So relax on this count.