Most Pidyon HaBen's take place in the afternoon. Do you say Tachanun at Mincha which is immediately preceding a Pidyon HaBen? (sources)
If the Pidyon Haben and its festive meal will take place immediately after davening, Tachanun is not recited (Ishai Yisrael 25:26 quoting Rav Shlomo Zalman Auerbach).
However the Mishnas Yehoshua brings in the name of the Sova Semochos that you do say Tachanun at a Pidyon HaBen