is tax return needed for B2?

for my parents' previous B2 visa, I provided with them the previous years' tax return along with other supporting documents, this time, I cannot find my 2008 tax return copy, wonder if this is really needed at all, cause from my parents, when they went for interview, the visa officer didn't really look at the stack of paper documents they brought with them - I do have W-2 form, my banking statements, I-134 etc.