If I am daveniing in a shul and that minyan is not not saying tachnun that day, for a Yartzeit or some other reason should one say tachnun B’tzina or not. If I say tachnun is it a din of Al Tifrosh Min Hatzibor
It is controversial, most poskim say that you may, but others say that it is better to go outside and say it, or not to say it all.
Tefilah Khilchoso 15 ftnt. 37* in the name of R’ Moshe Feinstein, and R Eliyashiv zt”l that one may say it and it isn’t being haughty because not saying it is again the halacha. Yabia Omer O:CH 11-9, Sefer Halacha Berurah O:CH 131-39, and ftnt 36, however in Halichos Shlomo (Tefilloh) 13 ftnt 7, R’ S. Z. Auerbach zt”l however says that he should go outside and say tachnun, unless it is established in this shul that on this day they don’t say tachanun.