In a lot of of the multi-vitamins on the market, it's an integral part. My Pregnacare vitamins state that they are suitable throughout pregnancy and breastfeeding, so on that basis I would say that folic acid certainly can't do any harm after 12 weeks.
Interestingly, there is an article online today in one of the papers about a study that has suggested that not only is Folic Acid good for preventing spina bifida, but also could potentially be linked to a decreased risk of a child having autism. As with all these studies, further research is needed, but it's an interesting read and does make you think about it from the findings.