Let me start by saying that we're firm believers in honesty always being the best policy. That having been said, there are times when you need to keep your mouth shut to preserve the peace. For example, when she says do these jeans make me look fat, you bold face lie if you have to, and don't take time time to think about it or she'll know. So how do you know when to confess and when to keep the peace? Here are a few guidelines.
When it comes to something serious, like an affair, some guys think that keeping it a secret will help preserve the relationship. First of all, no matter how careful you are, she'll probably either figure it out, or someone else will and tell her. Sneaking around just makes it look like you think she's dumb enough not to notice, and now you are just insulting her on top of affair. If you cheat on your woman, be a man and fess up. There is no right time to break that news, but there are bad times, so don't do it when anyone else is around or when you have to leave soon to go somewhere.
Sometimes, when you are in a relationship with someone there are things about them that just drive you bonkers. But confessing to the fact that you think she chews like a cow too early on in a relationship is a bad idea. If you can get past it and make it work, then you may just decide it is worth living with how she chews to get the rest of the package.
Notice how little things get left unconfessed (at least early on), but big things need to get addressed right away. That's is pretty much the key to making things work. If you mess up, ask forgiveness. If she messes up, decide if the things you love about her are enough to let it go, or if you have to have a heart to heart about it.
“I don't get along with your parents,” may be something she notices every time you are with them, but actually saying it out loud can result in a completely different dynamic. Instead of confessing to the obvious tension, ask for helping in dealing with her parents so that things go as smoothly as possible even though it won't be perfect.
If you have a past that may catch up with you (criminal activities, etc...) you'll have to break the news at some point or there will always be something between you. If you were engaged before, she might want to find that out before agreeing to marry you. If you cheated to pass your finals in college, she probably doesn't need to know (although you really shouldn't have done that). Focus on what directly affects her when deciding what to spill.