I think this involves using the taste buds, as to whether or not your rub needs some modification. I've stumbled upon a few store-bought rubs here and there, and even have a few favorites. I always taste them first though, to get feel for what balance is going on.. you know... the four senses of taste... salty, sour, bitter, sweet. And let's not forget our friend umami.. but I digress..
Being more or less a southern boy though, I prefer my rub to have a pronounced sweetness. Maybe you do too? Either way, the general theme of a rub, as a finished product, can be sensed by tasting it before you use it.
Case in point.. There's a Lysander's chipotle rub that I love for ribs. It's discontinued now, but I did stock up on it. Problem though... it's a little too salty, generally speaking. I know one way to fix that... sugar. I've also been know to dash some more chili powder into it as well. I'm adjusting the balance of those taste senses to what I like.
Like a lot of things.. experience helps. And making good bbq is a craft like anything else, so there's some trial and error. But if you focus on the taste.. before it goes on.. you should be able to develope a sense for what the end product might be.
best of luck..