Depends somewhat on what cooker you are using. Anything vertical, it probably makes sense to have the fat cap on the bottom to help protect the meat from drying out due to the heat coming from below.
If so, it makes no sense to put rub on the fat cap.
Jim - Another transplanted Texan
Former KCBS CBJ
Large and Medium Big Green Eggs , Black 18.5" WSM, Blue Weber Performer - Stainless, Green Weber OTG Kettle , Brinkmann SnP Pro, and a Stainless UDS. One retired Portable Kitchen grill.
Red Thermapen, Maverick ET-732, EdgePro Apex Sharpener.
Avatar is the original 1951 Weber Kettle