Concrete curb

This is a judgment call. Generally if a gutter is too rough to ride in I think of it as an imaginary curb, and ride on the asphalt close to the joint. When I get to the corner, though, I cut in and ride nice and tight. It's on the corners where errors will make the big difference.

Maybe you could also consider doing an offset maneuver as you would with any temporary obstruction on a course during measurement. Obviously you can't do this on a 90 degree curve because as Pete pointed out, it would result in a pretty big error. However, it should be applicable on the straight(er) sections.

Oscar's question reminds me of a time when I measured a 5K course during the height of leaf-raking season, and there were huge piles of leaves in the way of a good measurement, including some of them right at the corners. I dealt with this in 2 ways-- sometimes I projected a point from the curb out into the street and then "copied" the curve at the corner (still out in the street) until I got around the (virtual)corner to a point corresponding to an accessible point of the real curb. (Oh well, it's easier to do than to describe).
Another, simpler way, is sometimes possible: just put your bike up onto the curb and measure along the top of the curb. You won't shortchange the course. I think the course I'm remembering didn't have curbs where that was possible; they were those sloping curbs that don't have a "top".