I get a company match on my 401k contributions, up to 4% of each pay check. I also max out contributions each year. As a result, I have to time my bi-monthly contributions so that I reach the $15000 limit on the dec 31 paycheck, or I leave matching money on the table. As a result, I usually adjust my withholding percentage downward when I get a raise or bonus so that my last contribution on dec 31 paycheck is at least 4% of my paycheck. Then Jan 1 I readjust my withholding percentage so I'll get back to using up the IRS limit by Dec 31 again. I talked with HR about a better way to handle this and they couldn't come up with anything. They also couldn't comprehend where all the money was coming from to make a full $15000 contribution. :
Making my contributions approximately equal throughout the year also helps with budgeting and cash flow issues. My paychecks are usually about the same amount each month.