Well, I haven't managed to pull off a morning and a night water change in the same day yet so the jury will remain out on that topic. :)
Water changes are not like dosing as you can only get to 100% of any level (and in practice, not really even that), so no I don't think there will be any unusual side-effects to slacking off - just the usual ones! LOL
I think you have to look at the long term as an average, or every missed water change is a failure. (No point in that.) So, on a 7 day week, I'm only shooting for 5 water changes. Not very consistent, except as an average over the long term, right? At the same time, I have two "extra" days in the week where I can do bonus water changes, or that I can use to make up for "skip days" during the week. Also not very consistent, except over the long term.
I'm just hoping one-a-day continues working out.....I was basically doing no water changes before.
-Matt