I am on the same boat as you are. But I will only tank one graduate course, either B or C. I don't think it matters to me now. However, I thought differently a couple weeks ago.
A couple weeks ago, I did not get two out of three fellowships that I applied (these two are less competitive than the third waiting one, but the first two themselves are crazily competitive). So i thought I had no chance of getting the third one. Then I was thinking of what if I apply next year and my grade looks bad with B or C in graduate courses. I was sweating.
Luckily more than a week ago, I found out that I won the third fellowship that I applied for (most prestige and most money out of the three so fuk YEAH!). So now I don't mind of getting bad grades as long as I don't fail it. If I do fail, I wouldn't matter much I think. I am all set for graduate school now!
So the moral of my story is that if you are going to use your undergraduate transcripts for future applications then try to get your grades up as much as possible.
My little 2 cents.