I could not have been more determined to breast feed and it didn't work out for me, so I think it's great that you are being open minded and realistic.
You may have absolutely no problems at all, but as you say, you never know. I thought formula was the devil's work before I had DD (was influenced very heavily by the NCT...) and really resisted it when DD wasn't feeding effectively. I felt horribly guilty when I 'gave in' to supplementing her on advice, but in the end it was the right thing (and only thing I could have done, tbh, in my own case as DD was failing to gain weight).
My advice to you if you are concerned about struggling with BF would be to get some proper breastfeeding guidance before you give birth. What a lot of BF counsellors tell you - that it's all instinctive, that the baby will guide you etc., is IME, total bollocks! It is a skill like any other, which can take time and patience to acquire. Several people have recommended the Ina May Gaskin book on breast feeding to me, which I intend to read now I'm having DC2. I'm also going to have some one-to-one BF 'lessons' with a local independent midwife before I give birth.
I wouldn't worry about having formula in the house as it's so easy to get some if you need it any time, and as another poster said you might be tempted to resort to it sooner if it's to hand. From my own experience my supply reduced even further as soon as the supplementation started, so (purely in my experience, I'm not an expert) I understand that it's best not to do it before six weeks when the supply is more stable. If you struggle in the first days, go to a BF counsellor straight away, and if she doesn't suit you try another one as they are not all trained the same way. In my area there were about six different BF clinics and some definitely better than others.
But, if things aren't working for you, there is really nothing wrong with formula. I slightly prefer Hipp Organic as the milk they use is (as it sounds) organic so the cows won't have had the same exposure to antibiotics and hormones as other intensively reared dairy cows. That is purely personal preference and it's slightly more expensive I think. Most of my friends used Aptimil.