I think for me it is salt content. Some have to much salt so I don't put on as much. if less salt you can cover it more.
That is kind of how I think about it....and - personal tastes. Some like more salt...some less. You can always add more salt - but you can't take it out!
That's how I think about it too; personal taste and the rub being used play the biggest factor in how much to use.
For ribs, butts & brisket I make my own rubs, which are fairly low in salt compared to other rubs. So, for those generally I let the meat take as much rub as will stick to it.
