I should always be open and honest in all my thoughts and feelings, no matter how much it hurts. You may seem to feel better after you vent your feelings, but the long term results may leave you feeling worse. On the other hand, if you are sharing because you care about your companion and want to build your relationship, then how you feel is just one consideration. Your companion's feelings and reactions will be important to you also. The caring spouse not only shares, but also asks and listens.
Timing is important in sharing feelings.
If my wife has had a bad day and is exhausted and discouraged when I get home, the last thing she needs is my criticism. Later in the evening, after I have helped her clean up and put the kids to bed, she may be better able to respond to my concerns. Remember that some things may be better left unsaid. I look back at the many times I was tempted to explode at my wife over petty things that seemed big at the moment. I am grateful for those times that I bit my lip and let them pass.