Can I use titanium axle bolts on a steel frame? I'm thinking of doing so for the non-corrosive properties of titanium. Weight isn't an issue.

The thing I'm not sure of is the hardness of the titanium vs steel.
Presumably you're talking about a Profile or one of the other BMX hubs with ti hop-up bolts available. Yes, there is no issue. Were a bolt loose and able to rub dynamically against the frame, they could in theory chew it up because they'll probably be way harder. But if you're riding around with loose axle bolts you'll have other much more imminent problems.