Is it recommended to clear the query plan cache
I would recommend against this.  You have done optimization on probably a small subset of queries, and over time you will see the improvement and performance gain.  But if you just do a clean refresh of the query plan cache you will blow all of the execution plans out of there, affecting more than just your optimizations.

I'd be willing to say the immediate performance degradation isn't worth the smaller gain of using your optimized plans right away.  But again, this is another "it depends".