Do I really need a new Tax ID # and if so what about Sec. 645?

by Jodi
(Lancaster, CA)

My mother recently passed, had a Living Trust and I'm the Trustee. I've been told I need a new Tax ID # so her investments can be sold to be divided among beneficiaries. She only has about $35,000 in investments. When I went to IRS Form SS4 to get the new Tax ID #, it asks "Trust filing as an Estate under Sec. 645~Check if yes"~Do I check yes? I've also been told I don't NEED the new ID #. What happens if I don't get one?