Hmmm, it's an interesting one.
I can't say that chasing other cars isn't something I've not done in the past, as on unfamiliar circuits or in an unfaimilar car it can help you work out the lines. That said, CrumbMC is spot on when he says you should always be looking ahead, and if that's through the car in front then so be it.
I think I would say that it very much depends on the situation, and that it's fine to do as long as you don't come to rely on it.