In my opinion, when it comes to polyamory, and most other things in life, honesty from the beginning is always the best policy. When my husband and I decided to open up to the poly lifestyle, we decided on a full disclosure agreement. It is something I recommend. Sometimes the truth can be painful or scary, and sometimes it may loose you a potential partner. But those who can deal honestly with each other have the best chance for long term happiness. Not dealing honestly from the beginning tends to lead to hurt feelings, resentment, and growing mistrust.
Trust is the easiest thing to loose, and the hardest thing to get back.