That doesn't mean I can cut the lock and take something any time the tenant gets behind on rent.

Safety or perceived safety   IE crime drugs homeless etc.

I agree with comments so far about adding safety measures but would also recommend adding something to the rental agreement waiving liability with pictures of clear issue.

I'm looking for a very clear cut system to make sure my reserves are only used for reserves, Capex is only used for capex, and maintenance is only used for maintenance.

This is the double edged sword of investing and self-employed.

But with managing guest experiences, I'd want to cut my teeth on that for a minute before I delegated such to another party.

Also, I would recommend periodic (3-6 months) walkthru of the property to validate safety and ensure tenant is following the rules.

But I wouldn't want to cut it that close.

As the owner you get draws so you cut yourself a check. 

The safety of using doing Lockbox Showings will depend on:1) Class of property - the higher they class of property, the higher the class of prospects and the less problems you will have2) All of the above is based on statistics - there may always be an "idiot" that innocently leaves the door unlocked.