I used to use underscores back in the day (mostly for folders only), but they wouldn't work in the same sort order on my Macs, so I switched to exclamation marks. Those seem to work the same on all platforms. I also use multiple exclamation marks if I need a hierarchy (rarely).
Exclamation marks have the additional advantage of being quite narrow.
And they give me an additional laugh once in a while when other people have to deal with my folder hierarchy.
Nobody seems to recognize the additional character and everyone is rather confused by how everything is sorted
BTW: Have been away for a while and it's nice to see this place is still lively.