I found it odd that my girlfriend would say out the cents, because personally I have always been rounding up, and never considered the cents, for example.

I try to keep two things in my head when filling a grocery cart, an accurate tally of dollars and a numberless sense of "where in the dollar the cents remainder is presently", like the position of a progress bar on a screen. I find a remembered visual progress bar does not distract me from the arithmetic, and when I spot one of those silly prices ending in .95 or .99 the bar resets to a tiny bit and I add a dollar. My mental progress bar generally jumps in approximate ~1/4 steps and when the total is accurate to within $2 with many items that is a win.