I only did it for one school, and that was only because they called me at home on a monday asking for an interview on thursday, and in the same breath, asked me to submit a secondary before my interview (which they hadnt previously asked for). they gave me 3 days to complete a secondary and get it to them for an interview on thursday. I would only do it if you're in a situation like this or you're nearing the deadline. Otherwise, regular or priority mail is fine.