On the Search for Boss FAQ page it is stated that in the future Yahoo will introduce a fee for searching at above 10,000 search queries per day:
"BOSS currently allows for an unlimited number of daily search queries at no cost. However, in the near future Yahoo! will introduce a fee structure for BOSS usage above 10,000 search queries per day. This will most likely not impact the majority of websites using BOSS..."
Yes, you will need to sign up with Yahoo to use the search.