1) The Shach Al HaTorah (gematrias) writes: לא יח ל דברו ככ ל סופי תיבות אלול לומר שבאלול קודם ראש השנה יעשה התרה לנדריו – “The end letters of the words, לא יח ל דברו ככ ל, spells out אלול to hint to the fact that in Elul just before Rosh Hashanah we perform hatoras nedorim”. This is cited in the Shaarei Teshuvah (Orach Chaim 581:1).
2) I spoke to a number of rabbonim about this and they all said there is no need. Personally, I also don’t think hatoras nedorim is needed. I think the main reason is, that when we say an hanhogah tova requires hatoras nedorim, we mean that when one takes on something which has halachic rhyme and reason to it, i.e. women hearing shofar on Rosh Hashanah, waiting six hours between meat and milk, then in order to stop one requires hatoras nedorim. However, as much as people may think wearing a white shirt is a mitzvah, the reality is, there is no halachic reasoning for it. Interestingly, R’ Yaakov Skoczylas told me, that he was once speaking to R’ Nevenzahl about a different shailah, and he remarked “where does it say in halachah you must wear a white shirt?!?”
