The more clarity you can give to the musicians reading the music, the better the reading will be. Nowhere is that more essential than when in a reading session like the Rockford Choirs Cabaret I do every year. We have a full band and allow just five minutes per song to read down and fix problems.
That goes for adding notes above or below, in whatever vernacular you can find. Some notes that made it into my parts in this year’s Cabaret:
“shaker…or something”
“Norah Jones groove” (not on a Norah Jones song)
“with frequent little snare fills”
“[modulation]” (in a drum part, as a lighthouse)
“My count off is bar 1 #awkward” (for a score that has an eighth-note pickup that’s not notated a a pickup bar)
“just go to town”
These are in addition to lyrics at every double bar line, and clear notation throughout – always finding the mix of specificity and simplicity to read down quickly. I also made sure to make a TACET page for every song a player was sitting out on, so that they know they aren’t just missing a page.
I like to keep my scores as simple as I can, with nothing unnecessary to the player; but especially in fast paced reading situations, I think the more you can offer the player, the better. Especially if it’s personal and lighthearted!