I put it on both sides of the mast and use the numbers only as quick reference points.
That way, when you let to off to go downwind, or just sitting (like waiting for a start - releasing the DH will keep the sail from flogging), you can tell your crew, "Downhaul to 6!" or something like that when it needs to go back on.
A lot more precise than having them just giving it a yank.
I have them everywhere, even on the Jib and Main Travelers. It just looks cool .
Well that and it is lot 'easier' when sailing with inexperienced crew. Like Matt said, instead of saying "ease the jib traveler"..... just tell them to set the darn thing to 3 1/2.. Or whatever.. It really help keeps me sane and gives them a more enjoyable experience as I am less frustrated over something I didn't explain as well as I thought I might. I also found that having everything labeled REALLY helps. It is easy to be basically speaking a foreign language on the boat to the ears of someone who has never sailed before... You can point at everything and give it a name on the beach, but I don't think it soaks in the same as having it labeled and easy to see when out on the water.
I cut my graphics myself... I bought the wife this little Cricut cutter so she could do some scrapbooking stuff. She just had to have one. I don't think she has ever used it, but I have nearly worn the thing plum out.
Then.. hanging out on the beach if you ever want to keep them guessing, just point out the most important adjustment on the boat..