Fully agree. For my CPL flight test I wore nice slacks, polished shoes, and a shirt and possibly a tie (it was a while ago now.) As FSO Griffo says, don't wear stripes on your shoulders or anything just dress a little like it's an interview (staying practical, you've got to fly an aeroplane after all.) First impression count for a lot and if you look like you know what you're doing not only will you get yourself in the right frame of mind but you'll get the checker in the right frame of mind as well.