The more time you spend in the street, the more comfortable you will be with it.

When I first started commuting I spent most of my time on the concrete gutter pan. After a month or so, I had moved over to just left of the seam about half the time. Now, I'm consistently 8-10" to the left of the seam. The exception is when lanes are very narrow (on many arterials here they have paved over the gutter pan and added lanes to the road, making, for example, a 5-lane road into a 7-lane road), where I occupy about 1/3 of the lane.

As mentioned though, your speed is a factor. My cruising speed is over 20mph (on 45mph roads). The faster I'm moving, the safer I feel. At this point, I would never choose to ride on the sidewalk.