If it's way off line with her tank top all day, or if it's hanging down her shoulder, then that looks pretty sloppy and isn't attractive. If it's just a little offline, then it's no big deal. I would say that's no more or less attractive.
Here's a tip: If you are wearing like a pink spaghetti strap tank top, wear a pink bra. That way the straps will be the same color and not as noticeable. Someone who just glance at you will think it is part of your tank top. Or you could always get a strapless bra.
Personally I think it's kind of tacky, I wouldn't wear it out somewhere nice- but if I'm just hangin around someone's house or not doing anything special I don't think it's too big of a deal. At least try and match your straps to your tank top!