Advice/Suggestions on Fire-safe Safes (and water-proof) (legal)

[selhars]

This is the only place I could this to ask this......

I was researching fire-safe safes and saw a suggestion that because they're airtight and moisture can build up the safe should be opened every couple of weeks (for 20 minutes.)

Is that true for all fire-safe safes?
Or, only a concern if the box is in humid conditions?

I want a fire/water proof box. ...to put legal-sized documents in but I sure don't want to have open it every two weeks.

Thanks for any suggestions.