As per usual, there doesn't seem to be a "standard" way to do this, but I've ended up using a way that seems to work with the players I use and leaves me files named the way I like. More specifically, tracks in a "box set" should all have the same album name, have rising track numbers, and the files are named in such a way that win explorer sorts them in order...
step 1: number tracks with the first track of each CD numbered cdnum*100 +1 (so track 1 of disk 2 is 201)
step 2: rename files based on tags using
%artist% - %album% - CD$left(%track%,1)-$right(%track%,2) - %title%
so that track above would be named some artist - some album - CD2-01 - some title
The only "gotcha" is for CD sets that have more than 9 CDs as you can't add leading zeros to the track number - and I want the file for CD2, track 1 to be named ...CD02-01... so it sorts properly in windows explorer. So I use the Tag->filename formula
%artist% - %album% - CD$left($num(%track%,4),2)-$right($num(%track%,4),2) - %title%
(works with sets less than 100, all CDs less than 100 tracks - easy to change to accommodate)
Of course, other folks like other filename formats - easy to change tag->filename formulas.
I hope this helps someone, and, of course, I'd be glad to hear from anyone who's come up with a better way to do this...