Front load everything for my wife and I.
IRA's - January, first market day of the year.
Deferred Plans - get front loaded as much as our checks allow, all complete by October 1. (no matching)
Taxable - January, first market day of the year. Based on estimated savings for the year, if we over shoot we can invest less in taxable the year after. We also usually end up making some additional income, which allows us to make another contribution in December or adjust our reserves as needed.
We cannot control market returns, but we can control how many stocks we own per quarter, and compound those distributions/dividends over and over.