yes you are required to do so. if you are not provided one, make sure you ask for one.
a W9 is an information form, not a reporting form.
a W9 is used by whoever hires you to fill out a 1099 MISC at year end
they are required to fill out a 1099 if they paid you at least $600 during the tax year
regardless of whether you get a 1099 form, you are required to declare your income
if you get a 1099, you'd declare it as 1099 Misc income
if you don't get a 1099, you'd declare it as "other" or "miscellaneous" iincome