There is a view that if you have a plan B, it implies that you are not really committed to Plan A. I don't subscribe to this view. I always have a Plan B and usually Plan C, D and E too.
I think this is particularly important if you are a start up guy. If you are running a large corporate merrily executing your plan A then you can pretty much see the warning signs well in advance. You have months to gradually adjust your course, lobby your colleagues, change your messages so by the time you get to the crunch point your plan B merges seamlessly with your plan A. In fact it looks like plan B was plan A all along.
But if you are a start up guy, major shifts happen literally overnight. A key supplier that accounts for 60% of your business pulls the plug. Your VC backer tells you they are fully committed one day and sells their stake in you the next. Your critical tech guy is poached by someone who will pay her in actual real money.
If you don't have a plan B in your pocket and you are caught flat footed it can be Game Over. So, you execute plan A flat out. But the very second something goes wrong and you have to shift your strategy, you do it. You don't go in to a lengthy strategic review. You reach in to your pocket, pull out plan B and start executing that instead.
I am quite comfortable with doing this. I think it comes from my "tai chi" mindset that I apply intuitively to most things I do. Yin and Yang. Always looking for balance. Always ready to shift weight from one leg to the other to make sure I am properly balanced before I strike.