You should receive yiur W-2 by the end of January.
First the values on the paycheck and W-2 almost always differ somewhat and the IRS wants an exact match. (For example: Your paycheck (or paystub) will show your total wage. The W-2 box 1 will show the *taxable* wage, that can be different, and the non-taxable part will be reported in other boxes on the W-2.)
This can delay any refund by weeks.
You also will not know what to enter for the various boxes on the W-2.
Employers had until Jan 31 to issue the W-2. If it is different you will probably have to amend which takes about 4 months to process.
IRS regulations require you to either attach a copy of the W-2 to your mail tax return or have it in your possession if you e-file. The IRS can ask you to send them a copy of the W-2 even if you e-file so you must have it.
If you mail file then you are required to attach a copy of the W-2 to the tax return or the IRS will return it to you unprocessed. While you are not required to submit W-2's with e-filed returns, you still must have the W-2 in your possession since the IRS can ask for it when they review your e-filed return.
If you do not receive your W-2 by February 14, then you can use your paycheck and file a 4852 form, but you must follow the IRS rules in order to do so.
[Paraphrased from IRS rules]
Can you file without having received your W-2? Yes, but only if you have not received it by the end of February, and you have contacted your employer and attempted to obtain it, and you still have not received it then you contact the IRS (see the link below), then if the IRS is not successful in getting your W-2, you can use form 4852 in place of your W-2. But you MUST speak to the IRS before doing so. Otherwise, you cannot file before you receive your W-2.
people who file using their last check stub frequently have to amend, have their refunds delayed, or get notices from the IRS about issues. besides filing too soon does no good. the return just sits on the TT server until the IRS opens e-filing