There are some superstitious folks, so I tend to stay on the safe side to use よん and なな wherever possible. These are most 100% used when counting down as well, so it's good practice.
i asked my japanese native friend about this a while back, and the answer was essentially "whichever". neither is wrong, for よん/し or なな/しち, and people will understand you regardless. so don't worry about it :D
It‘s a matter of speed as well. If I‘m counting to 10 really fast, I stumble upon よん and なな. On the way down from 10, it‘s the other way round. しち doesn‘t fit well with the flow… It depends on the sound of the next/previous number.
Maybe I‘m not alone with that and that‘s even the reason for the difference mentioned by ギョルギ九十三…
