It shouldn't be a problem in most cases. Some people don't like writing it on there and just choose not to.
The IRS wants it on there (as well as the type of payment one is making) in case your check is separated from the paperwork, but it usually processes normally. And the check still has your name and address.
In any case, there's nothing really to do except see what happens. You can watch your online bank account, and your bank statement will be proof that they cashed it.
If at some point you want to be sure it was applied correctly to your IRS account, you can download an account transcript at the IRS website. But it may take a while to show up there.
http://www.irs.gov/Individuals/Get-Transcript
Yes
No
