Recently I've heard from quite a few people whose writing scores were raised from 6.5 to band 7 after they asked for a re-mark. This suggests that some examiners are a bit too cautious when marking, especially at the higher levels.
There are no guarantees when you ask for a re-mark. However, if you think you did well in the writing test, if you got higher scores in the other three parts, and if you missed your target by only half a band, it might be worth a try.
Please share your experiences of re-marking in the comments below.