If there is danger to the beard e.g. working as a mechanic, then it is prudent to loosely tie it from underneath only, but the way, the beard is tied now a days i.e. totally glued to the skin to give it a trimmed-beard look, is totally wrong. The use of fixo or a thread to tie the beard is totally wrong. The beard of a Gursikh should be natural and open. Bhai Sahib Randhir Singh jee has written a whole article on this subject - Sachiyaan Daariyaan.
Kulbir Singh