Is it normal for my Cane Corso to be wary of strangers?

Totally normal and desirable in the breed. The Cane Corso is a guardian by nature — initial wariness of strangers is part of his temperament, not a flaw. The problem would be if he showed uncontrolled aggression. What's key: continuous socialization, positive reinforcement when he ignores or accepts visitors, and teaching him a clear 'OK' cue that indicates the person is trustworthy.