Cracks in a steerer in general are in the "very very bad" category, but in this one case of a vertical crack in the key slot, I'm with the "ride it" camp. (Not that reality is decided by popular vote!)
If the crack has not progressed past the slot for the headset keyed washer, then it almost certainly never will. As long as (as has been mentioned already) the stem cone/wedge doesn't tighten near the slot.
As to the question of how to know where the crack ends, in case you want to drill a stop-hole: The dye-penetrant method works very well (like magic really) if you follow the directions carefully. DP kits are cheap and kinda fun to use, definitely easy enough for amateur mechanics as long as you're patient and clean the part well enough. Details were posted to CR recently, check the archives.
Mark Bulgier
Seattle, WA USA