I'm against apexing on city streets. There are curbs there, often square ones. Now, a 90 deg turn on a lonely road, different matter. The main reason for not apexing in town is crap in the corner. Look at the ground in the corner at an intersection. It's covered with all matter of crap. I've known several guys who's bike slid right out from under them because they thought they'd apex the corner, and I know of one guy who's peg hit the curb. Broke his leg, totaled the bike. Doesn't mean you have to walk it around the corner, just don't feel like you have to ace the corner.
Back to open roads, I never look at the apex anyway, I set my line up, then look through the corner. As Ridge, Mat and Justin said, just look through the corner, and your peripheral vision will tell you how close you came. Plus a buddy following you can be a real help. Remember to accelerate slightly through the corner too, helps keep your bike grounded.