St. Louis cut...Rub ???'s

No, it will just infuse more of the rub flavor for you. If it is a real spicey rub be careful not to over do it. I have done long rubs in the fridge with good results.

The longer you plan on leaving the rub on prior to smoking the less salt the better, in my opinion.

If you let a salty rub sit for more than about 12 hours it can start tasting "brine-y" (sp?).

This is 100% true.

If your rub contains a bit of salt, even 12 hours can give you the briney (sometimes called "hammy") taste to the ribs.

Ruined quite a few racks before learning this lesson.

Now, I only rub about a half hour before, while the pit is warming up.

Ribs only have, as the thickest part, 1" from top to bottom. You don't need hours like a pork butt or brisket for your rub to do the work.

To the specific question of 48 hours, I would say this is just too long, even with little to no salt. Again, from my early disasters, too early will "leech out" a ton of moisture from the ribs, which is then a double whammy: dry and hammy.

Just my .02, so YMMV

Good luck!!