Hello Adam,
For what its worth ny own view is that photographing at twilight will give you much brighter images than you might expect, almost like daytime, not convincing at all. If you want to give a truer impression of twilight I would wait until nearly astronomical twilight (when there is still a little light from the set sun in the sky) and include the north or western horizon in your shot. This sometimes gives your image a nice "glow" from the hidden sun which suggests true twilight.
Astronomical twilight can be calculated by visiting http://aa.usno.navy.mil/
An example of the effect I am trying to describe is available at:
http://www.nightfolio.co.uk/subpages/as002.html
This photograph was actually made at night, but has a twilight feel to it using the technique described above. Hope this is relevant.