Depending on what's in your rub...it's sometimes a lot better to rub just prior to or a couple hours before smoking the ribs. If your rub has a high salt content, rubbing and letting them sit overnight can cause the ribs to be "hammy".
Sorry it's not an answer, but it depends on what the rub is, how you smoke your ribs and if you like the results. What you want is repeatability of your own success.
Have you done it both ways? Notice a difference.
Same goes for a mustard slather. Some swear by it, others don't.
For me, I don't rub my ribs more than 1 hour ahead. I have a rub with some salt in it and any salt on pork overnight tends to change how I like mine.
Others LOVE theirs done overnight.
I'd say do both. Do one rack overnight, one rack the hour before and see if there is a difference. Usually on the overnight, you'll need to add more rub on before you put them in as the rub tends to moisten up a lot.
Last time I did spares all I added was salt & pepper, smeared a light coat of homemade sauce on them at the end of my smoke and got rants & raves, ooohhh's & aaahhh's. I usually do rub tho.
Point is, it's all subjective, agreeing with Smoking & Tom.
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