Is it important to dress in formal clothing for a postdoc interview?
Personally, I can't think of a single 'genuine' post-doc I've known over the years who had enough mental capacity left to even begin worrying about what to wear.

On a more serious note though, based on my experience within the field of computer science in The Netherlands (seeing as regional differences do matter), taking some care not to pick an indecent or dirty pair of clothes should be the minimum. Suits would be way overdressed.