Me? I always tell the person! But so many people don't and it amazes me. Are they blind or do they just ignore it because they don't want to embarrass the person (or themselves)?
Just this morning, while picking up a coffee, standing in a small queue. A couple of people down, there was a lady with her top label hanging out. The people behind her didn't say anything! I couldn't just stand there and ignore it, so when she walked past I told her and she gave me a genuine thanks.
People are always so grateful when you point it out, and personally I'd be really annoyed it one of my friends didn't tell me.
So - to those of you who don't - in future, do the right thing!