Like everyone else has said, a few Ws won't hurt you, no matter what they are in. When you have a lot, that is a different story.
I have a W in gen chem 2 lab. I've been invited for interviews at all 4 of my schools, and I've been accepted to 2. The two schools I have already interviewed with did blind interviews (they don't see your application beforehand, it is supposed to make it unbiased). They first one I didn't mention it, simply because I didn't think about it. The second I did.
I told them there was a W on my transcript for gen chem 2 lab. He kind of laughed and waved it off. I continued with why it was there. I got in, so I don't think it was a problem at all.
I'll probably only mention it at my other 2 interviews if they bring up something about it.
They understand that things happen and classes get dropped. I wouldn't worry about it at all unless there was a trend...