I have experienced both paths... Having a Plan B and not having it...

• If you don't have a Plan B, then you focus on the main plan is 100% and you can push with all your energy, without distraction... The bad thing is if your Plan A fails, you are left empty-handed...
• If you do have Plan B, you are doing the Plan A more relaxed, without pressure, and that can be actually a good thing... When I was doing one of my businesses like that in the past, I have the best results... But, on the other side, you can go in "too relaxed" area and ruin your work...

In conclusion... We have to do somewhere in the middle... :)