Don't forget about liability either danreller. If you lose a master key you'll have to rekey all your properties...And heaven forbid you don't and someone gets into a property without obvious signs of forced entry. I've worked for several large property managment firms and the 2 largest(Equity Residential and Theiss and Talle) have strict policies against masterkeying apartments. They both use the same method you use now...a locked box for duplicate key control.
A personal rant follows: Do Not Duplicate stamps are a legal butt covering method only now. Those "convenient" mall engraving shops that also cut keys for you will copy anything you bring in if they can buy a blank from Corbin or Ilco in bulk. I've discovered multiple unauthorized copies of the Best keys that we use at the college and we're in the middle of legal action with at least two of these places who claim it was done by an employee who no longer works there and of course they have policies in place to prevent this from happening. END of pointless rant.
All that being said, its going to be cheaper in the long run for you to get an account with Maintenance Warehouse and buy locks and keys through them and just key all your properties as you have been. It would be more convenient for you to use a single manufacturer for all your properties. And I've always found Schlage to be more "jiggle resistant" than any Kwikset/Titan/Weiser product.