Should I favor using a param or a scalar function result within a SELECT?
It sounds like a situation where you could potentially fall prey to - or benefit from - parameter sniffing. In a nutshell, SQL Server will use parameter/variable values when compiling an execution plan in order to determine optimal index usage (based on column statistics). Depending on the value that's passed in, you could get very different execution plans, and very poor performance if the "wrong" value is used later. (A nice article about it.).

I'd personally calculate the date within the procedure using a scalar function, unless there's a need for whatever is calling this procedure to specify the date itself. Then you could always use the scalar function in the procedure to fall back on a default value. As long as parameter sniffing isn't giving you surprising performance changes, it's mostly just a matter of application design.