The hard and fast rule is to do whatever it takes to keeps you safe.
The first thing is to ride with lights at night and reflective gear.
Generally, what I do is, if theres a wide enough shoulder I will use it. If there isn't, I take the lane. Being in a corner of a lane is a problem since motorists will try to sneak past and can't always judge the room needed to do so safely (at least a 3 foot margin from you). The side of the road is generally bad as debris tends to collect there, so you increase your risk of punctures and what not.
Note that there may be legal things to do -- The Washington state bicycling guide says cyclists must be as far right as it is safe not as far right as is possible (my personal policy also follows this). Check with your local area to see what the legal policy is (if any).
To quote the Washington state bicycling guide:
"Washington State law requires cyclists
to ride as far to the right as is safe,
not as far to the right as is possible.
There is a crucial difference. Assess the
situation to decide how far towards the
center of the lane you need to travel
to be safe versus unduly delaying
motorized traffic. Taking the full lane
is generally safest when traveling for
short stretches on lower speed roads
(20-25mph). On higher speed roads, it
may be safer to reduce your speed and
stay further to the right rather than
compete with faster vehicles for the
You may find it necessary to take
the lane in the following kinds of
• when traveling at traffic speed and
you need to prevent motorists from
inadvertently cutting you off;
• when descending a hill and you
need extra space (for reaction time)
due to your speed;
• when lane width does not permit
a motorized vehicle behind you to
• when road conditions (potholes,
road debris or parked cars) preclude
you from riding farther to the right.
Be aware that many motorists neither
understand that you have the right to
take the lane in these situations (or at
all), nor may they see road condition
hazards which you are trying to avoid."
Another thing to note is that part of safety (aside from abiding from the law) is dealing with harassment. From the same guide: "Most motorists are courteous and happy to share the road with cyclists, but
unfortunately, the small minority of drivers who are rude or threatening
stand out. Harassment can make a commute unenjoyable or even dangerous.
Fortunately, it rarely occurs. If harassed, try to keep your cool and remember
that your safety is the priority. You will rarely convince an irate motorist to share
the road, and besides, you don’t want to provoke a person unstable enough to
harass you in the first place. Your best bet is to develop your riding skills, know
your rights, ride legally and try to keep calm in the event you are harassed.
Noting a car’s license plate, description and occupants may help when reporting
an incident to the authorities."