Myrrha,
With the right correlation between the playing card meanings and the tarot card meanings, there shouldn't be a problem with skewing the reading away from an undesirable message. I've found, for the most part, that the tarot just tends to give the cartomantic assignments more depth and layers of understanding.
Actually, I think it's easier to use both systems. I've combined the old way I learned a long time ago, the "tarot way" I learned fairly recently, and a system I created for reading the Major Arcana into the cards. I thinks it's easier because it comes from the my own knowledge and reading experience.
I have a theory that the playing card meanings handed down from generation to generation might have been developed the same way. Undoubtedly, they went through many permutations as they were passed along. I think there's something wonderfully organic about drawing upon your own ancestry, knowledge, and experience to make your readings sacredly yours.