The W-2 you receive is a document that is also being filed with the IRS. The IRS will compare your reported W-2 amounts with what they have on file and if they don't match, you return could be rejected or you come under personal IRS scrutiny.
While figuring W-2 amounts from the last paycheck of the year is, technically, foolproof, you'll end up with a mismatch if the W-2 misreports your income. I just discovered today that my W-2 for 2009 reported my taxable income as $9.25 less than what could be figured by my last pay stub of 2009. Now, that may not have affected the amount of my tax return/payment, but it is a mismatch, and would've invited scrutiny. By using actual W-2 amounts, everything looks good to the IRS right away.
Incidentally, I went back and reviewed my 2008 W-2 and compared it with my last pay stub of 2008, and the two agreed perfectly.
I am not prepared to take the chance that my 2010 W-2 won't match with my last pay stub of 2010, so I will wait until I actually get the W-2 in my hands. Which according to payroll, should be January 19th!