Being open as you start a relationship is the best idea. When you do this there will always be something strong that will always fuel your inner being as you progress with what you two have started. Being open is good but there are some things that you are not supposed to be fully open about. This is because it might destroy your foundation. This does not mean that if you talk about it that you will not end up together but all you should know is it might be what will make your argument heat up and sometimes take time before you solve.
For one you should not talk about your EX weakness or the past relationship as what he will do he/he will start question your relationship as what will come into his mind will be if he/she is telling me this what does he tell her friend about me and if things go south before I make them look north what will he/she have said about me this weaken your love and what will follow is you will argue and the next min he/she will tell you to go tell so and so about it.
Something else that you should not discuss and what you should not tell the person you are dating is the number of people whom you have slept with, this should only be your secret if you want what you have to last for long as what you may tell him/her may make him/her judge you wrongly either you are naïve or he/she end up thinking that he/she will not be enough for you and this shake what you both have for each other and what will happen later is you will always wonder what happened.
Keeping something to yourself doesn’t mean that you don’t value love it means that you love him/her so much that you don’t want to lose her/him. Choose wisely what you talk with him/her.