Is it okay to clear SQL Server Buffer every weekend or month end?
It's almost always a good idea to set max server memory setting to some value lower than the total amount of memory available, especially if there are other applications running on the same server (IIS in your case). The actual number depends on total amount of memory and on memory requirements of those other applications.

This setting is important to prevent the various applications/services competing for memory (causing memory starvation), which in turn can cause performance problems.

With 32 GB available, I'd subtract from it 3 GB for the OS, then subtract whatever number is used by IIS and other applications, if any, and set the max server memory to the remainder.

There is no need to periodically clear the SQL Server buffer pools, just the opposite.