I applied Proofide to the top and bottom on a regular basis, initially more frequent than later.
IIRC, Brooks instruction for applying Proofide are to apply the proprietary wax to the top and the bottom when first applied, but for subsequent applications only to the top.
I presume that is to avoid building up wax on both sides so that the leather can breathe out any moisture through one flat side with only a light, single application of Proofide.
Trivia, I bought my Brooks saddle and leather grips in the Honey colour, not because it matched my bike well (it was actually a very good match but a bit flashy for me) but so that I could create a custom dark brown by soaking the saddle in neat foot oil (buy it from a saddlery or tack store which supplies the horse racing crowd).