In general, this is the list of types, from best (safest) to worst (not safest):
?), when both the nil and non-nil case are both handled (e.g. code is written) and tested (e.g. somebody tried it)
?), when the nil and non-nil case are 'eyeballed' to 'probably work'.
Generally, always be thinking about how to move higher up the safety tree. Improvements from 3 to 1 are possible in some cases, and case 4 should be eliminated in favor of one of the safer patterns. Improvements from 3 to 2 are always possible, but are often too much work to be practical.