What are some of the negative effects that lying to one's partner can have on a relationship?

Is honesty really the best policy when it comes to one's life partner? I hear people say that it's better to live with one's guilt rather than tell one's partner if there has been a one time betrayal. But wont guilt over lying eventually take a toll on the relationship? Is it better to work through the issues in the relationship and make the relationship an honest one, or can a relationship survive long term when there are lies involved...even lies that are intended to avoid immediate problems and prevent causing pain to one's partner? Everytime my partner tells me they love me, I wonder if they will still love me "if they knew" a secret I've been keeping from them, but I don't want to cause any pain with my secret. Any advice?