[eluser]Michael;[/eluser]
You do not. If you need a function across multiple controllers it will need to be placed in a library. I recommend you create a "common" library that allows you to store these functions in. Works best for these types of cases.
If you end up with a few related functions then you should probably create yourself a new library for those functions ... like for example if you have 3-4 functions specifically for date/time manipulation.
Michael