In my own case, I make roughly quarterly elective deferrals based on my quarterly net profit, up to the elective deferral limit of $18,000 plus $6,000 catch-up deferral.  I then make by employer contribution as soon as practical after the end of the year when I know the actual amount of my net profit.  Given that retirement investments are made under the assumption that the investments will grow on average, contributing as early as is practical give the investments more time to grow.

Note that to make an elective deferral, you are required to have made the deferral election by the end of the year.  However, the deposit of your elective deferral contribution can be made after the end of the year.  In my case, I make the maximum permissible elective deferral, so I make sure that I have in my 401(k) file forms dated no later than December 31 that total my maximum elective deferral.