Personally my high savings rate means we just pay for erratic expenses out of cash flow, and save a little less that month.
Then our spending jumps around on a graph of expenses (April spikes as taxes are paid, for example), but I have a line that shows the twelve month average, so I know what my average spending is, and annual spending.
Then some months are below, some are above, but I don't really care, I just pay whatever comes up, and save what I can.
Others more methodical than I may try to plan out things by saving monthly for the tax bill they know is coming, or saving for the next vacation, or saving in a car replacement fund. I just pay for things as they come up.