If you can not keep the room your cage is in over 76-78 all year round I do not believe you will be able to get away with just flexwatt. I keep my room between 70-72 all year round and it will sometime drop to 68ish on really cold nights. So I use an RHP.

Aaron are all of your cages stacked? J/w Could the flexwatt from the other cages be assisting with keeping your ambient temps up?