Strictly speaking, 0.15% should be pronounced point one, five percent, but I wouldn't bother correcting anyone for saying point fifteen percent, because I would know what they mean. I don't see the point in being overly pedantic and all it would do is annoy people.
By the way, I would pronounce 0.0015% as, nought, point, oh, oh, one, five percent or nought point double zero, five percent.