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The best place is going to be publication 15 - http://www.irs.gov/publications/p15/ar02.html
But this won't really be updated as quickly as one would hope so I would also recommend subscribing to "e-news for Payroll Providers," see link here - http://www.irs.gov/Businesses/Small-Businesses-&-Self-Employed/e-News-for-Payroll-Providers
here is recent withholding guidance for 2013 - http://www.irs.gov/uac/Newsroom/IRS-Provides-Updated-Withholding-Guidance-for-2013
OK, I review publication 15, and I have already subscribed to e-new for Payroll Providers, so I should be all set?
I would think so yes. This would ensure you have been alerted to any tax law changes and potential changes to withholding tables.
You should also periodically check the Form 941 instructions online to see if there are any changes.
Great! Thanks for your help. And I'll remember to rate your services accordingly :).